Objective: The authors examined patterns of substance use disorders and psychiatric and medical comorbidity in all male veterans treated in Veterans Affairs (VA) medical centers during a one-year period Methods: A national discharge abstract data base was used to derive point prevalence rates of psychiatric and medical disorders for 539,557 inpatients treated in VA medical centers in fiscal year 1991. Results: Nearly one-quarter of all male veterans treated in the study year bad a substance-related diagnosis. The most prevalent substance use disorders were alcohol dependence (87.7 percent) and cocaine dependence (17.5 percent), and the most frequent psychiatric diagnoses among veterans with substance-related diagnoses were depression and personality disorders. Digestive disorders were more prevalent among veterans who abused substances than among veterans who did not, and veterans who abused substances were also more likely to have multiple bospitalizations and longer index hospital stays. Conclusions: Substance use disorders and comorbid psycbiatric syndromes are common in male veterans treated in VA hospitals. Veterans with substance-related diagnoses may have longer hospital stays and more frequent hospitalizations than veterans without substance use disorders.