In fiscal year 2004, the federal government made or guaranteed about $84 billion in loans for postsecondary education through two loan programs--the Federal Family Education Loan Program (FFELP) and the Federal Direct Loan Program (FDLP). Under FFELP, private lenders fund the loans and the government guarantees them a minimum yield and repayment if borrowers default. When the interest rate paid by borrowers is lower than the guaranteed minimum yield, the government pays lenders special allowance payments (SAP). Under FDLP, the U.S. Treasury funds the loans that are originated through participating schools. Under the Federal Credit Reform Act (FCRA) of 1990 the government calculates, for purposes of the budget, the net cost of extending or guaranteeing credit over the life of a loan, called a subsidy cost. Agencies generally update, or reestimate, subsidy costs annually to include actual program results and adjust future program estimates. GAO examined (1) whether re-estimated subsidy costs have differed from original estimates for FFELP and FDLP loans disbursed in fiscal years 1994 through 2004, (2) what factors explain changes between re-estimated and original subsidy rates--that is subsidy cost estimates per $100 disbursed; and (3) which federal costs and revenues associated with the student loan programs are not included in subsidy cost estimates. Appended are: (1) Comparison of Fiscal Year 2006 FDLP and FFELP Re-estimated Subsidy Costs per $100 Disbursed, by Loan Type and Cohort; and (2) GAO Contacts and Staff Acknowledgments. (Contains 5 figures, and 13 tables.)