Two pairs of monozygotic twins discordant for gender identity are described. The first set are 8-year-old males, one of whom enjoys doll play, cross-dressing, and taking the role of a female in fantasy games. His brother shows no feminine behavior, enjoys sports, and is more masculine in gestures and speech. The second set are 24-year-old females, one of whom wants to undergo sex-change surgery to male status. Her cotwin is a feminine woman desirous of marrying and bearing children. Differential childhood experiences are described for the cotwins. These highlight environmental influences, which may contribute to variances in masculinity and femininity, when genetic influences are held relatively constant.