Data for this report came from a study of genetic and environmental risk factors for common psychiatric disorders in white female-female twin pairs from the Virginia Twin Registry. This is a population-based register formed from a systematic review of all birth certificates from 1918 onward in the Commonwealth of Virginia. The size of both the total sample (2,060 individuals from 1,030 complete female-female twin pairs) and that portion of the sample affected (185 women who met criteria for alcohol dependence and 357 who met broad criteria for alcoholism) was substantially greater than samples in previous studies. The results of the study, which are consistent across different definitions of illness, suggest that genetic factors play a major etiological role in alcoholism in women. Further, contrary to a recent report, results suggest that the role of genetic factors in women is similar for both narrowly and broadly defined alcoholism. These results are also inconsistent with those of previous twin and adoption studies, which suggest that genetic factors play at best a minor role in the etiology of alcoholism in women. Several methodological differences between this study and previous investigations might account for these different findings. More research is needed to determine how heredity influences a woman's chances of becoming an alcoholic.