The purpose of this paper is to rebut the assertion that reassign ment or change of sex necessarily constitutes an extreme psychological hazard for individuals who have been assigned to the wrong sex at birth. Some doubt will also be thrown on the claim that psychological sex?normal or abnormal?develops entirely as a consequence of environmental psychological experience, like learning a language, and is neither innate nor influenced by hormones. Money (1955, 1963), Hampson (1955), Money et al. (1957), and Hampson and Hampson (1961), who carried out investi gations on cases of intersexuality, are responsible for comparing the role?that is, the psychological sex?to the learning of one's native language and asserting that the gender role is in agreement with the sex of rearing and not instinctively determined by chromosomes, gonads, or hormones. They concluded that reassignment of the sex of rearing was without doubt psychologically injurious and constituted an extreme psychological hazard. There is no doubt that the conclusions of Money and the Hampsons have profoundly influenced the treatment of wrongly assigned sex, particularly in North America, where the attitude is more rigid and where change of sex would cause greater social reactions than in Europe. Although Dennison (1965) agrees with Money, it has not been the experience of Cappon et al. (1959) or of mine (Armstrong, 1964). I find it difficult to accept that in man the psychological sex, or the gender role, is solely dependent upon psychological environmental factors, such as the sex of rearing, and indepen dent of hormones. Herbert and Michael (1965) and Michael et al. (1966) have shown that in rhesus monkeys the sexual perform ance of the males depends on the endocrine status of their female partners. Sexual variety of the male varied with the phases of the female menstrual cycle, and bilateral ovariectomy of the female suppressed or greatly reduced the sexual activity of the male ; this was readily restored after administration of oestradiol to the ovariectomized female. The administration of progesterone to females already receiving oestrogen resulted in a notable reduction in the ejaculatory capacity of their male partners. The types of cases which in my opinion can?and should? be reassigned without psychological hazard are particularly cases of hypospadias in boys assigned and reared as girls and cases of congenital adrenal hyperplasia in girls assigned and reared as boys. The former can be corrected by surgery and the latter by hormone therapy, and if normality can be achieved it is obviously preferable that the patients should be treated rather than that they should be allowed to continue their lives as a pathological abnormality. It has always been my practice to treat cases of congenital adrenal hyperplasia with hydro cortisone and reassignment of sex, if necessary, and I have never encountered any difficulties.