We examine the effect of the Medicaid program on the labor supply and welfare participation decisions of female heads of family. A key contribution is the development of a family-specific proxy for the valuation of Medicaid benefits. We find that Medicaid has strong and significant effects on labor supply and welfare participation that are negative and positive in sign, respectively, but which are concentrated in the tail of the distribution with the highest expected medical expenditures. We also find that the availability and level of private health insurance have very large effects opposite in sign to those of Medicaid.