This study investigated effects of San Francisco's Treatment on Demand Initiative, which was designed to increase substance abuse treatment capacity, on the host treatment system. Secondary data on substance abuse treatment admissions, from 1995 through 2000, were obtained from the Department of Public Health, Community Substance Abuse Services, San Francisco, California. Data on 73,988 admissions were retained and grouped by week of admission. Time series analysis was used to assess the effects of time and treatment on demand (independent variables) on weekly number of admissions, sociodemographic characteristics and types of treatment received (dependent variables). As a function of treatment on demand, the number of weekly new admissions significantly increased. Standard outpatient treatment and comprehensive care admissions constituted greater proportions of admissions after treatment on demand. Persons with a primary heroin, cocaine or alcohol problem constituted greater proportions of admissions, and first-time treatment clients constituted a smaller proportion of admissions. Findings suggest that a capacity expansion initiative can increase system wide admissions and redistribute admissions among modalities towards more comprehensive care treatments.