ALLI Allion Healthcare (MM)

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

o

Rule 13d-1(b)

x

Rule 13d-1(c)

o

Rule 13d-1(d)

CUSIP No. 019615103

1

Name of Reporting Person
Discovery Equity Partners, L.P.

2

Check the Appropriate Box if a Member of a Group (See Instructions)
Not Applicable

(a)

o

(b)

o

3

SEC Use Only

4

Citizenship or Place of Organization
Illinois

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person With

5

Sole Voting Power
None

6

Shared Voting Power
715,311

7

Sole Dispositive Power
None

8

Shared Dispositive Power
715,311

9

Aggregate Amount Beneficially Owned by Each Reporting Person
715,311

10

Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions)  o
Not Applicable

11

Percent of Class Represented by Amount in Row (9)
2.8%

12

Type of Reporting Person (See Instructions)
PN

CUSIP No. 019615103

1

Name of Reporting Person
Discovery Group I, LLC

2

Check the Appropriate Box if a Member of a Group (See Instructions)
Not Applicable

(a)

o

(b)

o

3

SEC Use Only

4

Citizenship or Place of Organization
Delaware

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person With

5

Sole Voting Power
None

6

Shared Voting Power
815,498

7

Sole Dispositive Power
None

8

Shared Dispositive Power
815,498

9

Aggregate Amount Beneficially Owned by Each Reporting Person
815,498

10

Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions)  o
Not Applicable

11

Percent of Class Represented by Amount in Row (9)
3.1%

12

Type of Reporting Person (See Instructions)
OO

CUSIP No. 019615103

1

Name of Reporting Person
Daniel J. Donoghue

2

Check the Appropriate Box if a Member of a Group (See Instructions)
Not Applicable

(a)

o

(b)

o

3

SEC Use Only

4

Citizenship or Place of Organization
U.S.A.

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person With

5

Sole Voting Power
None

6

Shared Voting Power
815,498

7

Sole Dispositive Power
None

8

Shared Dispositive Power
815,498

9

Aggregate Amount Beneficially Owned by Each Reporting Person
815,498

10

Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions)  o
Not Applicable

11

Percent of Class Represented by Amount in Row (9)
3.1%

12

Type of Reporting Person (See Instructions)
IN

CUSIP No. 019615103

1

Name of Reporting Person
Michael R. Murphy

2

Check the Appropriate Box if a Member of a Group (See Instructions)
Not Applicable

(a)

o

(b)

o

3

SEC Use Only

4

Citizenship or Place of Organization
U.S.A.

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person With

5

Sole Voting Power
None

6

Shared Voting Power
815,498

7

Sole Dispositive Power
None

8

Shared Dispositive Power
815,498

9

Aggregate Amount Beneficially Owned by Each Reporting Person
815,498

10

Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions)  o
Not Applicable

11

Percent of Class Represented by Amount in Row (9)
3.1%

12

Type of Reporting Person (See Instructions)
IN

Item 1.

(a)

Name of Issuer
Allion Healthcare, Inc.

(b)

Address of Issuer’s Principal Executive Offices
1660 Walt Whitman Road, Suite 105, Melville, New York 11747

Item 2.

(a)

Name of Person Filing
Discovery Equity Partners, L.P.  (“ Discovery Partners ”)

Discovery Group I, LLC, the general partner of Discovery Partners (“ Discovery Group ”)

Daniel J. Donoghue, a Managing Member of Discovery Group

Michael R. Murphy, a Managing Member of Discovery Group

(b)

Address of Principal Business Office or, if none, Residence
Discovery Partners, Discovery Group, Mr. Donoghue, and Mr. Murphy are all located at:

191 North Wacker Drive, Suite 1685, Chicago, Illinois  60606

(c)

Citizenship
Discovery Partners is an Illinois limited partnership

Discovery Group is a Delaware limited liability company

Mr. Donoghue and Mr. Murphy are U.S. citizens

(d)

Title of Class of Securities
Common Stock, par value $0.001 per share

(e)

CUSIP Number
019615103

Item 3.

If this statement is filed pursuant to §§240.13d-1(b) or 240.13d-2(b) or (c), check whether the person filing is a:
Not Applicable

(a)

o

Broker or dealer registered under section 15 of the Act (15 U.S.C. 78o);

(b)

o

Bank as defined in section 3(a)(6) of the Act (15 U.S.C. 78c);

(c)

o

Insurance company as defined in section 3(a)(19) of the Act (15 U.S.C. 78c);

(d)

o

Investment company registered under section 8 of the Investment Company Act of 1940 (15 U.S.C 80a-8);

(e)

o

An investment adviser in accordance with §240.13d-1(b)(1)(ii)(E);

(f)

o

An employee benefit plan or endowment fund in accordance with §240.13d-1(b)(1)(ii)(F);

(g)

o

A parent holding company or control person in accordance with § 240.13d-1(b)(1)(ii)(G);

(h)

o

A savings associations as defined in Section 3(b) of the Federal Deposit Insurance Act (12 U.S.C. 1813);

(i)

o

A church plan that is excluded from the definition of an investment company under section 3(c)(14) of the Investment Company Act of 1940 (15 U.S.C. 80a-3);

(j)

o

Group, in accordance with §240.13d-1(b)(1)(ii)(J).

Item 4.

Ownership

Provide the following information regarding the aggregate number and percentage of the class of securities of the issuer identified in Item 1.

(a)

Amount beneficially owned:   

Discovery Partners – 715,311

Discovery Group – 815,498

Mr. Donoghue –815,498

Mr. Murphy – 815,498

(b)

Percent of class:   

Discovery Partners – 2.8%

Discovery Group – 3.1%

Mr. Donoghue – 3.1%

Mr. Murphy – 3.1%

The foregoing percentages are based on 25,910,625 shares of Common Stock of the Issuer identified in Item 1 outstanding as of November 4, 2008, as reported in the Issuer’s Quarterly Report on Form 10-Q for the quarterly period ended September 30, 2008.

(c)

Number of shares as to which the person has:

(i)

Sole power to vote or to direct the vote   

None

(ii)

Shared power to vote or to direct the vote    

Discovery Partners – 715,311

Discovery Group – 815,498

Mr. Donoghue – 815,498

Mr. Murphy – 815,498

(iii)

Sole power to dispose or to direct the disposition of   

None

(iv)

Shared power to dispose or to direct the disposition of   

Discovery Partners – 715,311

Discovery Group – 815,498

Mr. Donoghue – 815,498

Mr. Murphy – 815,498

Item 5.

Ownership of Five Percent or Less of a Class

If this statement is being filed to report the fact that as of the date hereof the reporting person has ceased to be the beneficial owner of more than five percent of the class of securities, check the following   x .

Item 6.

Ownership of More than Five Percent on Behalf of Another Person

Not Applicable.

Item 7.

Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on By the Parent Holding Company or Control Person

Not Applicable

Item 8.

Identification and Classification of Members of the Group

Not Applicable

Item 9.

Notice of Dissolution of Group

Not Applicable

Item 10.

Certification

By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect.

February 13, 2009

Date

DISCOVERY GROUP I, LLC,
for itself and as general partner of
DISCOVERY EQUITY PARTNERS, L.P.

Michael R. Murphy*

Signature

Michael R. Murphy, Managing Member

Name/Title

Daniel J. Donoghue*

Signature

Daniel J. Donoghue

Name/Title

Michael R. Murphy*

Signature

Michael R. Murphy

Name/Title

*By: /s/ Mark Buckley

Mark Buckley
Attorney-in-Fact for Daniel J. Donoghue
Attorney-in-Fact for Michael R. Murphy

Exhibit 1

Joint Filing Agreement dated as of February 13, 2009, by and among Discovery Equity Partners; L.P.; Discovery Group I, LLC; Daniel J. Donoghue; and Michael R. Murphy

Exhibit 2

Power of Attorney of Daniel J. Donoghue, dated as of April 28, 2008

Exhibit 3

Power of Attorney of Michael R. Murphy, dated as of April 28, 2008