OBJECTIVE: The purpose of this study was to determine whether female homosexuality is familial and whether it is cofamilial with male homosexuality. METHOD: Subjects included 84 homosexual and 79 heterosexual female probands recruited through newspaper advertisements. Probands were asked about their siblings' sexual orientations and were asked for permission to contact siblings to confirm their reports. RESULTS: The authors were able to contact 60% of eligible siblings, and the information they provided about their sexual orientations confirmed that probands' reports were highly accurate. Homosexual probands had a significantly higher proportion of homosexual sisters according to four criteria for rating siblings' sexual orientations. Homosexual probands also had a higher proportion of homosexual brothers; however, this difference was not significant. CONCLUSIONS: Female homosexuality appears to be familial. Further research is required to resolve the question of whether female and male homosexuality are cofamilial.