IN THIS PAPER, we will report briefly on a series of male twins where at least one of each pair was homosexual. One object is to report the number of concordant and discordant, monozygotic (MZ) and dizygotic (DZ) pairs observed. A second object is to examine the frequency of homosexuality in twins per se. One of the twin pairs was from a family deserving special attention. Among 14 siblings, there were three sets of male MZ twins. Two of these three sets of twins were concordant for homosexuality; in the remaining pair both twins were heterosexual. The results of our study of this family will be presented in detail. The use of twin studies as a means of disentangling some of the interaction of nature and nurture is sufficiently well known for it to be unnecessary to give any elaborate explanation here.1,2Briefly, differences in MZ pairs provide