A survey evaluated the lifetime and current prevalence of mental disorders in 501 patients seeking assistance with alcohol and other drug problems at an addiction research and treatment facility. Information was gathered using the National Institute of Mental Health Diagnostic Interview Schedule (DIS) and computer diagnoses were generated according to DSM-III criteria. Four fifths (78%) of the sample had a DIS lifetime psychiatric disorder in addition to substance use, and two thirds (65%) had a current DIS mental disorder. Excluding the unreliably diagnosed generalized anxiety disorder, the most common lifetime disorders were antisocial personality disorder, phobias, psychosexual dysfunctions, major depression, and dysthymia. Patients who abused both alcohol and other drugs were the most psychiatrically impaired. Patients with DIS psychiatric disorders had more severe alcohol and other drug problems. Barbiturate/sedative/hypnotic, amphetamine, and alcohol abusers were the most likely to have a DIS mental disorder.