Uniformed Services Medicare Subvention Program Act - Directs the Secretaries of Defense and of Health and Human Services (HHS) to jointly establish a subvention program to provide the Department of Defense (DOD) with reimbursement from the Medicare program under title XVIII of the Social Security Act for health services provided to Medicare-eligible covered military beneficiaries who agree to receive such services through the managed care option of the TRICARE program (a DOD-managed health care program). Makes program enrollment voluntary. Requires the Secretary of Defense to waive the TRICARE enrollment fee for program participants for whom Medicare reimbursement may be made.

Directs the HHS Secretary to make monthly payments to DOD from the Federal Hospital Insurance Trust Fund and the Federal Supplementary Medical Insurance Trust Fund representing appropriate reimbursement amounts. Provides for the determination of such amounts.

Directs the Secretary of Defense to: (1) maintain DOD health care efforts for Medicare-eligible covered military beneficiaries; (2) estimate, for the first fiscal year of the subvention program, the amount expended by DOD for FY 1997 for providing health care items and services to such beneficiaries; and (3) establish monthly targets of the number of such beneficiaries for whom reimbursement will not be provided to DOD.

Requires the Comptroller General, for each program year, to report to the Secretaries and the Congress on the extent to which costs under the TRICARE program and the Medicare program have increased as a result of the subvention program. Requires the Secretaries to take necessary steps to offset any excess costs and prevent future excess costs, including: (1) suspension or termination of the subvention program; (2) adjustment of the payment rate; or (3) adjustment of DOD maintenance of effort requirements.