An open clinical trial was conducted to compare the effects of rapid (1-day) admission with slow (14-day) admission to methadone maintenance on pretreatment attrition, retention during treatment, and other outcomes. One hundred eighty-six illicit opioid users eligible for methadone maintenance were randomly assigned to rapid admission or slow admission, with 93 subjects assigned to each group. The random assignment produced two groups that were similar on 22 personal variables. All subjects admitted to treatment were followed for 1 year. Follow-up interviews were obtained with 155 (98%) of the 158 subjects admitted to treatment. During the period from initial contact to medication, only 4% of the rapid admission subjects but 26% of the slow admission subjects dropped out. The risk of dropout during slow admission was 6 times that during rapid admission. A higher percentage of rapid admission subjects, 43%, than of slow admission subjects, 39%, remained continuously in treatment for 1 year, but the difference was not significant. The two subgroups that remained in treatment for 1 year did about equally well on measures of illicit drug use and social performance. The findings indicate that pretreatment attrition can be markedly reduced by prompt medication, and that prompt medication does not adversely affect retention during treatment or other outcomes.