The number of participants in the SSI program grew by 1.1 million from 1987 to 1993. This paper examines the role of Medicaid on the SSI participation decision. I use the rapid growth in average Medicaid expenditure as a proxy for its value. OLS estimates of Medicaid’s effect may be biased because of omitted variables bias and measurement error. I therefore apply two-stage least squares to estimate Medicaid’s effect, using average Medicaid expenditure for blind SSI recipients as an instrument. These estimates show that rising Medicaid expenditure significantly increased SSI participation among adults with low permanent incomes, explaining 20% of the growth.