Methadone maintenance clients who used benzodiazepines were compared with other methadone maintenance clients on a range of drug use and psychosocial treatment outcome measures. Despite being on higher methadone doses, benzodiazepine users were more likely to have recently injected, to have used cocaine and amphetamines, to have borrowed or lent used needles and syringes, and to have reported polydrug use in the preceding month. Benzodiazepine users also exhibited higher levels of psychopathology and social dysfunction than other methadone maintenance clients. It is concluded that benzodiazepine-using methadone maintenance clients are a dysfunctional subgroup of the methadone population, who may require more clinical intervention than other clients.