BACKGROUND: Physician income varies threefold among specialties. Lower incomes have produced shortages in primary care fields. OBJECTIVE: To investigate the impact of government policy on generating income differentials among specialties. DESIGN AND PARTICIPANTS: Cross-sectional analysis of the 2004 MEPS. MEASUREMENTS: For outpatient care, total payments made to 27 different types of specialists from five types of payers: Medicare, Medicaid, other government (the Veterans Administration and other state and local programs), private insurance, and out-of-pocket payments. For inpatient care, aggregate (i.e., all-specialty) inpatient physician reimbursement from the five payers. RESULTS: In 2004, physicians derived 78.6% of their practice income ($149,684 million, 95% CI, $140,784 million-$158,584 million) from outpatient sources and 21.4% of their income ($40,782 million, 95% CI, $36,839 million-$44,724 million) from inpatient sources. Government payers accounted for 32.7% of total physician income. Four specialties derived > 50% of their outpatient income from public sources, including both the lowest and highest paid specialties (geriatrics and hematology/oncology, respectively). CONCLUSIONS: Inter-specialty income differences result, in part, from government decisions.