Unmarried men aged 25--35 were recruited through newspaper and radio advertisements for a family study of sexual orientation. Fifty predominantly homosexual and 50 predominantly heterosexual men comprised the index subjects. The test, questionnaire, and interview battery is described. Subjects were asked about the sexual orientations of their relatives. The homosexual index men report a significant excess of homosexual brothers and more distant male relatives but not of homosexual sisters or female relatives. Moreover, the percentage of homosexual brothers increases with the Kinsey rating of the index subjects. Overall, about 25% of brothers of homosexual men are reported also to be homosexual.