Mixed gonadal dysgenesis (MCG) is a rare intersexual disorder, characterized in most cases by the presence of a testis and a contralateral streak gonad; in some cases the contralateral gonad may be rudimentary not having differentiated into an ovary or into a testis and in other cases it may be absent. A personal case was recently studied by the authors in some detail: A 51-year-old individual reared as a female with primary amenorrhea, short stature, subnormal intelligence, male type habitus, hirsutism, moderate breast development, ambiguous external genitalia with a 5 x 2 cm phallus, labia majora with scrotal type skin, and a urogenital sinus. Internal genitalia consisted of a hypoplastic vagina and uterus, a streak gonad on the right and intraabdominal testis, fallopian tube, and epididymis on the left side. The dysgenetic testis contained a gonadoblastoma and the streak a gonadoblastoma and dysgerminoma. A buccal smear was chromatin negative and karyotype analysis revealed XO/XY mosaicism. Thyroid and adrenocortical function was normal. A plasma testosterone level of 0.15 mu-g% rose to 0.45 mu-g% with 3 days of HCG administration. With bilateral gonadectomy the plasma testosterone level fell to 0.06 mu-g% and there was no response following HCG administration. 109 cases of MGD and 2 cases of unilateral gonadal dysgenesis reported in the world literature between 1937 and 1973 were reviewed. On the basis of their clinical and pathological features, the latter two patients were excluded from the category of MGD. In 15 of the 109 cases of MGD, a gonadoblastoma obscuring the gonad of origin was opposed by a dysgenetic gonad with or without a gonadoblastoma or no gonad. In these cases the presence of a testis was not proven histologically and their clinical-pathological features were similar to those found in pure gonadal dysgenesis. Of the remaining 94 cases of MGD, 72 had testicular tissue on one side and a streak or embryonic gonad on the other while 22 had a testis and no contralateral gonad...