ABSTRACT. Objective: The Danish Longitudinal Study on Alcoholism was designed to identify antecedent predictors of adult male alcoholism. The influence of premorbid behaviors consistent with childhood conduct disorder (CD) and attention-deficit/hyperactivity disorder (ADHD) on the development of alcohol misuse was examined. Method: Subjects were selected from a Danish birth cohort (9,125), which included 223 sons of alcoholic fathers (high risk) and 106 matched sons of nonalcoholic fathers (low risk). These subjects have been studied systematically over the past 40 years. They were evaluated in their teens (n = 238), later as adults at age 30 (n = 241), and more recently at age 40 (n = 202). At 19-year/20-year follow-ups, an ADHD scale was derived from teacher ratings and a CD scale was derived from a social worker interview. At 30-year and 40-year follow-ups, a psychiatrist used structured interviews and criteria from the Diagnostic and Statistical Manual of Mental Disorders, Third Edition, Revised, to quantify lifetime alcoholism severity and to diagnose alcohol-use disorder. Of the original subjects, 110 had complete data for the two childhood measures and the adult alcoholism outcomes. Results: In this smaller subsample, paternal risk did not predict adult alcohol dependence. Subjects who were above a median split on both the ADHD and the CD scales were more than six times more likely to develop alcohol dependence than subjects who scored below the median on both. Although the two childhood measures were correlated, a multiple regression showed that each independently predicted a measure of lifetime alcoholism severity. Conclusions: ADHD comorbid with CD was the strongest predictor of later alcohol dependence. (J. Stud. Alcohol Drugs 70: 169-177, 2009).