The authors examined the relationship of comorbid non-substance use psychiatric disorders to preadmission problem status and treatment outcomes in 278 methadone maintenance patients. Recent admissions were assigned DSM-III-R Axis I and II diagnoses according to structured diagnostic interviews. The Addiction Severity Index was administered at admission to assess past and current substance use and psychosocial problems and again 7 months later. Treatment retention and month 7 drug urinalysis results were also obtained. Across substance use and psychosocial domains, participants showed significant and comparable levels of improvement regardless of comorbidity. Comorbid participants received more concurrent psychiatric treatment which may have accounted for the lack of differential improvement among groups. Nevertheless, psychiatric comorbidity was associated with poorer psychosocial and medical status at admission and follow-up and participants with the combination of Axis I and II comorbidity had the most severe problems. Admission and month 7 substance use were, for the most part, not related to psychiatric comorbidity, although there was a trend indicating more treatment attrition for participants with personality disorders.