Plasma estrone sulphate ( E1S ) and estrone (E1) concentrations were determined in healthy postmenopausal women and in postmenopausal women with endometrial cancer, matched for body weight, age, and years since menopause. E1S levels (mean +/- SD) were significantly higher (P less than 0.05) in cancer patients with normal weight (511 +/- 200 pg/ml) than in control subjects (303 +/- 99 pg/ml). E1S levels were also higher in obese cancer patients (691 +/- 328 pg/ml) than in obese control subjects (610 +/- 139 pg/ml). Both cancer groups showed similar plasma E1 levels as compared with their respective controls. The E1S /E1 ratio was higher in both groups of cancer patients than in control subjects. These data suggest that estrogen conjugates should be taken into account during studies on estrogen balance and endometrial cancer.