Three homosexuals from an unselected series of twins have been briefly described. Two pairs were monozygotic and a third dizygotic. In all three the co-twins showed no evidence of any sexual deviation. These observations suggest that factors other than genetical ones may play a significant part in the aetiology of homosexuality. In the one adequately documented case, the mother's attitude, commencing prenatally and continuing in a favourable setting, is considered to be the important factor.