Nuclear progesterone receptor (PR) concentrations were measured in 40 samples of endometrial adenocarcinoma and in 13 samples of normal endometrium taken from the same hysterectomy specimen. The levels of PR, expressed in pmol/mg DNA, were correlated with clinical and histopathologic characteristics of endometrial cancer and with the concentrations of progesterone in ovarian blood. The results suggest that PR levels might serve as a additional qualitative and quantitative prognostic risk factor in patients with endometrial cancer.