The aim of this study was to assess the efficacy of methadone compared with buprenorphine maintenance therapy in heroin-dependent patients over a treatment period of 18 weeks. Subjects were randomized to receive either methadone or buprenorphine in a comparative double-blind study and consisted of 164 heroin-dependent male patients who met the DSM-IV criteria for heroin dependence and were seeking treatment. The 164 subjects included 41 patients in 1-mg, 41 patients in 3-mg, and 41 patients in 8-mg dosage group of buprenorphine, and also 41 patients in the 30-mg dosage group of methadone. The mean age was 31.4 years for total buprenorphine group and 33.7 years for methadone group (the mean age differences in 4 groups were not statistically significant). Subjects received buprenorphine at a dose of 1, 3, or 8 mg per day or methadone at a dose of 30 mg per day and were treated in an urban outpatient clinic, offering a 1-hour weekly individual counseling session. Days retained in treatment were measured. Completion rates by buprenorphine dosage group were 29.3% for the 1-mg dose group, 46.3% for the 3-mg dose group, 68.3% for the 8-mg dose group, and 61% for the 30-mg methadone dose group. Retention in the 8-mg dose group was significantly better than in the 1-mg dose group (p=.00041) and in the 3-mg dose group (p=.045); other comparison (1 mg dose with 3 mg dose) was not significant. Methadone group was significantly better than 1mg buprenorphine dose group (p=.004), but was not significantly different from 3 mg buprenorphine dose group (p=.18) or 8 mg buprenorphine dose group (p=.49). The results support the efficacy of buprenorphine for outpatient treatment of heroin dependence and seem to indicate that the highest dose (8 mg) of buprenorphine was the best of the three doses of buprenorphine, and also support the superiority of 30 mg of methadone compared to 1 mg dose of buprenorphine for Iranian heroin-dependent patients to increase their retention in treatment.