We examined data from a large cohort of homosexual and heterosexual females and males concerning their siblings' sexual orientations. As in previous studies, both male and female homosexuality were familial. Homosexual females had an excess of homosexual brothers compared to heterosexual subjects, thus providing evidence that similar familial factors influence both male and female homosexuality. Furthermore, despite the large sample size, homosexual females and males did not differ significantly from each other in their proportions of either homosexual sisters or homosexual brothers. Thus, results were most consistent with the possibility that similar familial factors influence male and female sexual orientation. However, because results conflicted with those of some other studies, and because siblings' sexual orientations were obtained in a manner likely to yield more errors than in these other, smaller studies, further work is needed using large samples and more careful methods before the degree of cofamiliality of male and female homosexuality can be resolved definitively. We also examined whether some parental influences comprised shared environmental effects on sexual orientation. Scales attempting to measure such influences failed to distinguish subjects with homosexual siblings from subjects with only heterosexual siblings and, thus, did not appear to measure shared environmental determinants of sexual orientation.