Data from a 1980 discharge survey of general hospitals were analyzed to determine the prevalence of cases with coexisting diagnoses of mental and substance abuse disorders. Twelve percent of total cases (or 208,000 episodes) had dual diagnoses, a proportion similar to that found in another analysis of 1985 data. Of these cases, 55.5 percent had a primary diagnosis of an alcohol or drug disorder, most commonly alcohol- or drug-induced organic brain syndrome. Thirty-four percent of cases with a primary diagnosis of mental disorder had depressive neurosis, 24 percent had psychosis, and 19 percent had personality disorder. An additional 18.9 percent of all cases had two or more mental or substance abuse disorders. Dual-diagnosis cases had a shorter mean hospital stay than cases with mental disorder only.