A phenotypic female patients with 46,XY true hermaphroditism and clitoromegaly was found at laparotomy to have normal müllerian structures with a right streak gonad and a left dysgenetic ovotestis that contained a gonadoblastoma. Karyotypes of peripheral lymphocytes and cells from labial skin and both gonads were all 46,XY, so an unrecognized mosaic or chimera was unlikely. Tissue from both gonads was studied in vitro and estrogen and testosterone production measured. The streak produced 186.6 pg/mg/24 hr of estrogen and 78 pg/mg/24 hr of testosterone, whereas the dysgenetic ovotestis produced 85.8 pg/mg/24 hr of estrogen and 184.6 pg/mg/24 hr of testosterone (more than double the testosterone produced by the streak). Comparisons between these data and those previously published suggest that dysgenetic streak gonadal tissue is more active steroidogenically than postmenopausal ovarian stroma.