[This paper develops a rigorous partial-equilibrium analysis of the determinants of the marginal welfare cost (MWC) of taxes on labor earnings. It shows that four key parameters interact to determine the magnitude of MWC. Using aggregate data and plausible ranges of values for the parameters, MWC can vary from under 10 percent to more than 300 percent of marginal tax revenue, suggesting that, given available evidence, we cannot estimate MWC with much precision.]