The debate over privatizing Medicare stems from a fundamental disagreement about whether privatization would primarily generate consumer surplus for individuals or producer surplus for insurance companies and health care providers. This paper investigates this question by studying an existing form of privatized Medicare called Medicare Advantage (MA). Using difference-indifferences variation brought about by payment floors established by the 2000 Benefits Improvement and Protection Act, we find that for each dollar in increased capitation payments, MA insurers reduced premiums to individuals by 45 cents and increased the actuarial value of benefits by 8 cents. Using administrative data on the near-universe of Medicare beneficiaries, we show that advantageous selection into MA cannot explain this incomplete pass-through. Instead, our evidence suggests that insurer market power is an important determinant of the division of surplus, with premium pass-through rates of 13% in the least competitive markets and 74% in the markets with the most competition.