Studies of patients admitted to public mental hospitals have consistently found high rates of comorbid substance use disorders. We sought a better understanding of this comorbidity among psychiatric inpatients, in particular differentiating two groups of "dual diagnosis" patients, (1) those with independent mental disorders complicated by substance use disorders, and (2) those with psychoactive substance use disorder-induced organic mental disorders (PSUD-induced OMD). The diagnoses of 435 consecutively admitted inpatients from an inner-city catchment area were ascertained with the Structured Clinical Interview for DSM-III-R (1987 Inpatient Version [SCID-P]), modified to describe more accurately the relationships between psychiatric syndromes and substance use. More than half (55.9%) of these psychiatric patients had current substance use disorders. Over half (53.6%) of these "dually diagnosed" patients had no lifetime history of an independent mental disorder, but rather had psychiatric syndromes related to psychoactive substance use. The dual diagnosis subgroups differed on treatment history, principal psychiatric diagnoses, and the types of substances used. The results underscore the heterogeneity of dual diagnosis patients admitted to psychiatric hospitals and the potential importance of differentiating among these subgroups.