The endogenous estrone (E1) and estradiol (E2) levels (in pg/g tissue) were measured in 7 postmenopausal patients with a hyperplastic endometrium, in 3 with an atypical adenomatous hyperplastic endometrium and in 13 with a carcinomatous endometrium. These tissue concentrations were compared with the E1 and E2 concentrations in plasma (pg/ml) and with data from a control group of postmenopausal women with atrophic endometria. The tissue levels of both steroids showed large variations and there were no significant correlations with their plasma levels. In the hyperplastic and the atypical adenomatous hyperplastic group the mean E2 tissue level was higher compared with the mean E1 tissue level, despite the excess of E1 over E2 in peripheral plasma. In the carcinomatous group the mean E1 tissue level was higher, although not significantly, than the mean E2 tissue level. There were no significant differences between the E1 and E2 tissue levels in the three different pathological groups as compared to the atrophic control group. We conclude that it is unlikely that estrogens alone play a crucial role in the development of a pathological endometrium.