Urinary estradiol production rates (PBE2) were found to be elevated in 6 consecutive men with gonadotropin-producing tumors (4 with choriocarcinoma, and 2 with ectopic gonadotropinproducing tumors of stomach and lung). The PBE2 ranged from 63 to 605 μg/day (1½- to 15-fold elevated) and correlated with gonadotropin production and tumor mass. To determine the origin of the elevated E2, 3H dehydroepiandrosterone sulfate (DHEA-SO4) was given iv and its conversion to urinary E2 determined by reverse isotope methods. The transfer constant ρBUDHEA-SO4→E2 ranged from 0.5 to 11.0% and increased as the PBE2 increased. Control cancer patients had ρ values of 4 to E2 accounted for all of the increased PBE2 in patients making greater than 100 μg/day of E2. Primary tumor tissue and metastatic nodules from 1 patient with choriocarcinoma were incubated with either 3H-DHEA or 3H-DHEA-SO4 and conversion of these substrates to estrone and estradiol was shown by reverse isotope dilution and crystallization methods, confirming the tumor tissue as the site of these steroid transformations. Primary tumor tissue from the 2 patients with ectopic gonadotropin-producing tumors were similarly shown to convert DHEA-SO4 to E2, thus behaving like trophoblastic tissue, despite histologic differences. Increased estradiol production from circulating DHEA-SO4 represents an additional biochemical marker that might be useful in detecting the presence of gonadotropin-producing tumors. These data provide further evidence that gonadotropin-producing tumors behave like normal placental tissue irrespective of histological appearance, and thus may represent an interesting form of retro-differentiation.