One method of investigating the genetic etiology of alcoholism is to study individuals who were separated soon after birth from their alcoholic biological parents. The author and his colleagues conducted a series of such studies in Denmark; they concluded that, despite little exposure to the alcoholic biological parent, the sons of alcoholics were about four times more likely to be alcoholic than were the sons of nonalcoholics. They also found that having an alcoholic biological parent did not increase the sons' risk of developing psychiatric disorders other than alcoholism. After reviewing the results of four other studies that resemble the Danish series, the author discusses two investigations of the possible mode of transmission of alcoholism and describes further evidence for the proposed diagnostic category of familial alcoholism.