This study examines the use of nonpsychiatric services by mentally ill persons following the receipt of specialized mental health care, frequently referred to as the "offset effect." A total of 9,761 persons enrolled during 1975 in the Columbia Medical Plan, a prepaid group practice in Columbia, Maryland, were studied over a 5-year period. Enrollees were classified into three groups: Treated--mental disorder diagnosis in 1975 and specialized mental health care in 1975; Untreated--mental disorder diagnosis in 1975 but no specialized mental health care in that year; and Comparison--neither mental disorder diagnosis nor specialized mental health care in 1975. The nonpsychiatric utilization for these groups was compared for 1973-1977. Specialized mental health care appears to have a short-term effect on nonpsychiatric utilization by attenuating the peak in use. Mentally ill persons without specialized mental health care in 1975 also reduced their use of nonpsychiatric services in 1976-1977. The utilization changes were more likely to occur in primary care departments, rather than nonpsychiatric specialty care departments. A diagnosis of mental disorder in either 1973 or 1974 was associated with a larger offset effect.