To assess the impact of a New York City supportive housing program on housing stability and preventable emergency department (ED) visits/hospitalizations among heads of homeless families with mental and physical health conditions or substance use disorders. Multiple administrative data from New York City and New York State for 966 heads of families eligible for the program during 2007-12. We captured housing events and health care service utilization during 2years prior to the first program eligibility date (baseline) and 2years postbaseline. We performed sequence analysis to measure housing stability and compared housing stability and preventable ED visits and hospitalizations between program participants (treatment group) and eligible applicants not placed in the program (comparison group) via marginal structural modeling. We matched electronically collected data. Eighty-seven percent of supportive housing tenants experienced housing stability in 2years postbaseline. Compared with unstably housed heads of families in the comparison group, those in the treatment group were 0.60 times as likely to make preventable ED visits postbaseline (95% CI=0.38, 0.96). Supportive housing placement was associated with improved housing stability and reduced preventable health care visits among homeless families.