General population data from the National Comorbidity Survey are presented on co-occurring DSM-III-R addictive and mental disorders. Co-occurrence is highly prevalent in the general population and usually due to the association of a primary mental disorder with a secondary addictive disorder. It is associated with a significantly increased probability of treatment, although the finding that fewer than half of cases with 12-month co-occurrence received any treatment in the year prior to interview suggests the need for greater outreach efforts.