Retention of patients in methadone treatment was studied in a cohort of 238 heroin addicts who entered maintenance programmes between February 1986 and August 1987. All subjects had been assessed at a centralised unit and referred to one of two other units for maintenance. Of the ten client characteristics that we analysed, three--a history of imprisonment, a history of dependence on barbiturates or benzodiazepines and employment status at entry--were included with "clinic" and maximum dose of methadone in the Cox regression models. Allowing for the other four variables, the maximum daily dose of methadone dispensed during the study period was a highly significant predictor of retention (P less than 0.00001). With maximum dose stratified into three levels--less than 60 mg, 60-79 mg, 80+ mg--and with the lowest stratum used as the baseline, the relative risk (RR) of leaving treatment was halved (RR 0.47, 95% confidence interval [CI] 0.33-0.67) for subjects receiving 60-79 mg, and halved again (RR 0.21, 95% CI 0.12-0.38) for those who received 80+ mg. Clinic dosage policies contribute significantly to retention in methadone maintenance treatment. Clinics need to develop dosage policies in negotiation with individual patients.