Objective: Permanent supportive housing (PSH) programs are being implemented throughout the United States. This study examined the relationship between fidelity to the Housing First model and health service use among clients in PSH programs in California. Methods: Data from a survey of PSH program practices were merged with administrative data on service utilization to examine the association between fidelity to a benchmark program, the Housing First model, and health service use among 5,067 clients in 77 PSH programs. Regression analyses were used to compare inpatient, crisis and residential, and outpatientmental health service use between high-, mid-, and low-fidelity programs in a pre-post design. Results: During the preenrollment period, clients in mid- and high-fidelity PSH programs, compared with low-fidelity programs, used inpatient and crisis and residential services more but used outpatient mental health services less. Postenrollment, patients in high-fidelity programs showed the largest increase in the number of outpatient visits, followed by clients in mid- and low-fidelity programs: 71.6 versus 48.2 and 29.0, respectively. Conclusions: Clients in housing programs with higher fidelity to the Housing First model had greater increases in outpatient visits. Compared with lower-fidelity programs, higher-fidelity programs also enrolled clients who used fewer mental health outpatient services in the year before enrollment. Higher-fidelity programs may be more effective than lower-fidelity programs in increasing outpatient service utilization and in their outreach to and engagement of clients who are not appropriately served by the public mental health system.