The ability of human endometrium to synthesize estrogen from testosterone (T) was investigated. Normal.and malignant endometrial specimens were incubated in a complete nutrientmedium with 1 uC/ml (Ü10 pmol/ml) of [3H]T for 20 hrs. Various estrogens: estrone (E1), estradiol (E2), estrone sulfate (E2S), and estradiol−3−sulfate (E2S) were isolated from culturedtissue and medium. The capacity of aromatization, expressed in pmol of estrogen formed/g oftissue, of proliferative endometria was found to be significantly_higher than that of secretoryendometria (prol. n=12, 0.53 ± 0.21, vs sec. n=13, 0.15 ± 0.09/ x = s.d., P<0.005). In ninecancer endometrial specimens studied, the estrogen produced varied from 0.3 to 15 pmol/g oftissue.These studies represent the first evidence that human endoraetriura is capable of synthesizingestrogens from A androgens at a concentration similar to plasma level. The changes ofthe capacity of aromatization during the two phases of the menstrual cycle indicate that theestrogen synthesis in endometrium is apparently regulated by hormones. The presence of aromatasein cancer endometria may play an important role in promoting the cell growth in estrogensensitive endometrial cancer.