This study was conducted to determine whether the histopathology of the testis of the male transsexual would throw light on the pathogenesis of male transsexualism. Histological sections of testicular tissues obtained at sex reassignment surgery from phenotypic male transsexuals (n = 10, age 21-33 years) with XY sex chromosome constitution were studied by light microscopy. The patients were diagnosed on clinical and psychiatric evidence and had been on oestrogen therapy for 6-13 years. The significant histological findings in the ten subjects were: (i) Focal or normal spermatogenic activity associated with normal Leydig cell population in three cases, and (ii) Total absence of spermatogenic activity associated with reduced Leydig cell population in seven cases. We suggest that the observed histological features are due to refractoriness to oestrogen, and the iatrogenic effects of oestrogen superimposed on normal or altered hypoathalamo-pituitary function.