This study examines prevalence rates for DSM-III-R anxiety and affective disorders in three follow-up samples of opioid addicts who were treated with methadone maintenance. At least one anxiety disorder was diagnosed in 55% of the total sample. Affective disorders were found in 58%. At least one anxiety disorder coexisted with at least one affective disorder in 36% of the sample. The research demonstrates that opiate addiction in this sample is most often associated with other comorbid psychopathology. It suggests a need for thorough assessment for general psychopathology in opioid addicts entering addiction treatment, especially assessment for anxiety and affective disorders. It also suggests the need for treatment that focuses on diagnosed mental disorders in addition to drug counseling for the substance abuse disorder.