Twenty-two children with female external genitalia, testes, and 46,XY karyotype, most presenting with inguinal lumps or hernias, underwent bilateral gonadal excision (21 patients) or biopsy. On clinical and biochemical grounds, 13 were classed as androgen resistance (AR), 6 as testosterone biosynthetic defect, 2 as XY gonadal dysgenesis, and 1 as 5 alpha-reductase deficiency. The main pathological difference between the groups was that in AR the gonocytes were present in normal numbers, whereas in testosterone biosynthetic defect, gonocytes, though present in infancy, were rare or absent by 12 years. In all groups, Leydig cells, normally not recognizable between 1 year and puberty, were often present and could not always be attributed to a gonadotropin stimulation test. In four cases the tubules were more heterogeneous than those seen in control testes, but tubular atrophy and increased stroma were indistinguishable from those in age-matched cryptorchid controls. Small Sertoli cell nodules were found in three children, but no carcinoma in situ was identified. The epididymis, said to be absent in adult cases of AR, was consistently present but often cystic or atrophic. Oviduct-like structures were found in three cases of AR, and smooth muscle resembling round ligament in four. Because of the difficulty in distinguishing the types of male pseudohermaphroditism microscopically, we recommend that XY female children should undergo thorough endocrinologic investigation before orchidectomy.