The lifetime prevalence of DSM-III-R alcohol abuse and alcohol dependence and associated patterns of psychiatric comorbidity in the Ontario population aged 15-64 years are estimated from a survey of a representative household sample using the University of Michigan Composite International Diagnostic Interview (UM-CIDI). More than half (55%) of all individuals with an alcohol disorder have a lifetime comorbid disorder and comorbidity is more common in women than in men with an alcohol disorder. The odds of having other drug disorders and antisocial personality disorder are very high in individuals with an alcohol disorder compared to those without. Alcohol dependents, but not alcohol abusers, have significantly increased odds of mood and anxiety disorders compared to individuals without an alcohol disorder. Sociodemographic risk profiles, alcohol use patterns and course differ for alcohol abusers/dependents with and without a comorbid disorder. Primary anxiety and drug disorders are risk factors for subsequent alcohol disorders.