Medical histories of the 15,924 male twin pairs in the National Academy of Sciences-National Research Council Twin Registry were examined to determine, within pairs, concordances for alcoholism and its medical end points. Prevalences per 1,000 among individual twin subjects were 29.6 for alcoholism, 4.1 for alcoholic psychosis, 14.2 for liver cirrhosis, and 2.1 for pancreatitis. Prevalences were similar to monozygotic (MZ) and dizygotic (DZ) twins. Prevalences in percent among co-twins of diagnosed subjects, that is case-wise twin concordance rates, were, respectively, by diagnosis: alcoholism: 26.3 (MZ), 11.9 (DZ); alcoholic psychosis: 21.1 (MZ), 6.0 (DZ); and liver cirrhosis: 14.6 (MZ), 5.4 (DZ). No twin pairs concordant for pancreatitis were found. The greater concordance for alcoholic psychosis and for liver cirrhosis among MZ than DZ twins could not be explained by the difference in alcoholism concordance between them. The difference in concordance between MZ and DZ twins persisted when, in addition, it was assumed that only half of the actually occurring cases of alcoholism and of each of the end points have been ascertained. These results provide evidence in favor of genetic predisposition to organ-specific complications of alcoholism and should serve to stimulate searches for the underlying biochemical mechanisms.