UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13G
(Rule 13d-102)

o

Rule 13d-1(b)

o

Rule 13d-1(c)

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Rule 13d-1(d)

CUSIP No.  G9618E 10 7

1.

Names of Reporting Persons. I.R.S. Identification Nos. of above persons (entities only)
John J. Byrne

2.

Check the Appropriate Box if a Member of a Group (See Instructions)

(a)

o

(b)

o

3.

SEC Use Only

4.

Citizenship or Place of Organization
United States

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person With

5.

Sole Voting Power
1,032,082**

6.

Shared Voting Power 
73,379**

7.

Sole Dispositive Power 
1,032,082**

8.

Shared Dispositive Power
73,379**

9.

Aggregate Amount Beneficially Owned by Each Reporting Person
1,105,461**

10.

Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions)  o

11.

Percent of Class Represented by Amount in Row (9)
10.3%**

12.

Type of Reporting Person (See Instructions)
IN

Item 1.

(a)

Name of Issuer
White Mountains Insurance Group, Ltd.

(b)

Address of Issuer’s Principal Executive Offices
80 South Main Street

Hanover, New Hampshire 03755

Item 2.

(a)

Name of Person Filing
John J. Byrne

(b)

Address of Principal Business Office or, if none, Residence
80 South Main Street

Hanover, New Hampshire 03755

(c)

Citizenship
United States

(d)

Title of Class of Securities
Common Shares

(e)

CUSIP Number
G9618E 10 7

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:   

See Schedule A hereto.

(b)

Percent of class:   

See Schedule A hereto.

(c)

Number of shares as to which the person has:

See Schedule A hereto.

(i)

Sole power to vote or to direct the vote   

(ii)

Shared power to vote or to direct the vote    

(iii)

Sole power to dispose or to direct the disposition of   

(iv)

Shared power to dispose or to direct the disposition of   

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   o.

Not applicable.

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

Not applicable. 

Date:  January 7, 2005

Date

By:  /s/ John J. Byrne

Signature

Name:  John J. Byrne

Name/Title