OBJECTIVE We assessed whether homeless adults entering substance abuse treatment in Massachusetts were less likely than others to enroll in Medicaid after implementation of the MassHealth Medicaid expansion program in 1997. METHODS We used interrupted time-series analysis in data on substance abuse treatment admissions from the Treatment 0Episode Data Set (1992-2009) to evaluate Medicaid coverage rates in Massachusetts and to identify whether trends differed between homeless and housed participants. We also compared Massachusetts data with data from 17 other states and the District of Columbia combined. RESULTS The percentage of both homeless and housed people entering treatment with Medicaid increased approximately 21% after expansion (P = .01), with an average increase of 5.4% per year over 12 years (P = .01). The increase in coverage was specific to Massachusetts, providing evidence that the MassHealth policy was the cause of this increase. CONCLUSIONS Findings provide evidence in favor of state participation in the Medicaid expansion in January 2014 under the Affordable Care Act and suggest that hard-to-reach vulnerable groups such as substance-abusing homeless adults are as likely as other population groups to benefit from this policy.