In order to obtain a rationale for surgical sex reversal in clinical practice, case reports of 149 patients with female pseudo-hermaphroditism due to fetal adrenal cortical hyperplasia have been reviewed and tabulated, with the inclusion of 5 new cases. The masculinization in female pseudohermaphroditism is usually progressive. The progression can be detected in childhood by increased bone age, advanced secondary sex characteristics and elevated 17-ketosteroid excretion. Of 53 patients over 15 years of age, 14 were considered to be male at birth but 2 were later "feminized" by clitoridectomy. The remaining 39 were reared as females, but 4 became so masculinized that it was later decided to regard them as males. Many of the others were adjusted poorly to their environment because of their masculine physical characteristics. Since clitoridectomy is irreversible, it perhaps had best be reserved until puberty for patients with feminine inclinations but without progressive masculinization, or for patients who will take suppressive doses of cortisone indefinitely. The decision as to sex in these individuals should rest on the degree and progression or suppression of masculinization, since social and psychologic sex patterns will follow the arbitrary "environmental" sex.