While methadone maintenance treatment (MMT) has been demonstrated to be an effective treatment for opiate dependence, its impact on the treatment outcome of other illicit drug abuse is not as clear. Using the initial urine drug screen (UDS) and follow-up UDS at 1, 6, 12, and 24 months, 167 patients consecutively admitted to MMT were evaluated for opiate, sedative (predominantly benzodiazepine), and stimulant (predominantly cocaine) use. Retention for the opiate only group was 97.32 days longer on average than for patients using opiates along with stimulants, sedatives, or both stimulants and sedatives. Patients abusing opiates only had the greatest decrease in drug use; however, MMT was also associated with decreases in cocaine and sedative use over the 24 month follow-up period. There was no evidence that patients "switched" their drugs of abuse with time in treatment. The negative impact of non-opiate drug use on outcome in MMT and its implications for treatment planning are discussed.