This study investigates the impact of methadone treatment on inpatient care admissions and mortality among 331 methadone-maintained patients compared with 1,483 similar untreated opiate misusers. The methadone patients manifested a lower mortality during treatment than the comparison group and those patients who had left their treatment. The annual incidence rate decreased from 1.4 inpatient care admissions per year for those who had stayed 0-1 year, less than 1 for those who stayed more than 2 years, and 0.3 for those who had stayed longer (>4 years) in methadone treatment. A similar decrease occurred in both sexes. The incidence rate decreased more among the HIV-negative than the HIV-positive patients. The low incidence of inpatient care during treatment may be due to treatment but may also be partly due to selection factors.