The stock of U.S. physicians at any point in time is modeled as a weighted average of the supply that a perfect cartel would produce and that would prevail under perfect competition. Estimation of a system of stock and income equations over the post-World War II period shows that, after holding constant demand and marginal cost conditions and accounting for gradual adjustment to changes in equilibrium, the weighting parameter has moved toward the competitive extreme since 1965. This rise in the degree of competition is estimated to have increased physician stock by 6%-20% and concomitantly decreased medical incomes by 19%-45%.