Male monozygotic cotwins of probands with Alcohol Abuse-Dependence (n = 85) were more likely than male same-sex dizygotic cotwins (n = 96) to report alcohol, drug, and conduct disorder problems. For women, rates of problem behavior did not differ between monozygotic (n = 44) and same-sex dizygotic (n = 43) cotwins. Opposite-sex dizygotic twin data (n = 88) revealed significant cross-sex transmission; alcohol problems were greatest among male cotwins of female probands. For men, proportion of liability variance associated with additive genetic factors was significantly greater when proband had an early (h2 = .73 +/- .18) rather than late (h2 = .30 +/- .26) age of onset. For women, heritability did not vary as a function of proband's age of onset, and the pooled estimate suggested little genetic influence (h2 = .00, SE not computable). Findings suggest that genetic influences may be substantial only in the etiology of early-onset male alcoholism.