OBJECTIVE: The authors seek to understand in general the sources of familial resemblance for alcoholism and in particular how parents transmit the vulnerability to alcoholism to their daughters. METHOD: The authors interviewed 1,030 pairs of female same-sex twins of known zygosity from the population-based Virginia Twin Registry and 1,468 of their parents. They examined a narrow definition of alcoholism, requiring tolerance or dependence, and a threshold approach that classified individuals either as unaffected or as suffering from one of three levels of severity of alcohol-related problems. Twin-family structural equation models were fitted to the observed tetrachoric or polychoric correlation matrices by using asymptotic weighted least squares. RESULTS: In the best-fitting model from both diagnostic approaches, 1) the familial resemblance for alcoholism was due to genetic factors, with the heritability of liability estimated at 51% to 59%; 2) genetic vulnerability to alcoholism was equally transmitted to daughters from their fathers and from their mothers; and 3) alcoholism in parents was not environmentally transmitted to their children. Assortative mating for alcoholism was found only for the broader definitions of illness. Genetic factors that influenced the liability to alcoholism were the same in the parental and twin generation for the narrow definition of alcoholism. When broader definitions were used, these factors, while substantially correlated, were not identical. CONCLUSIONS: The transmission of the vulnerability to alcoholism from parents to their daughters is due largely or entirely to genetic factors.