This study assesses the impact of catchmenting through comparing the use of an emergency room-mental health center complex by residents of a catchmented as opposed to sociodemographically comparable, but noncatchmented service area over the period of one year. While the range of care available to both areas was similar, the catchment was served by a unit funded under the Community Mental Health Centers Act and the noncatchment by a general community service. The patient populations derived from the homogeneous areas were sociodemographically and clinically comparable. Catchmenting was associated with a higher volume of care and greater accessibility. The availability of catchmented services was shown to modify existing patterns in the use of mental health services.