The prognostic value of clinical features, qualitative and quantitative pathologic characteristics and steroid receptor incidence was evaluated in 50 patients with Stage I endometrial adenocarcinoma. It turned out that many of these features were prognostically important. Estrogen receptor content was not significantly associated with prognosis in our material, but patients with progesterone receptor positive lesions had a better survival than those in which the tumors were progesterone receptor negative. Multivariate analysis also clarified that only three features in combination had independent significance: mean shortest nuclear axis, DNA index, and depth of myometrial invasion (in sequence of decreasing importance). The prognostic rule consisting of these features overshadowed the value of all other features investigated. The overriding prognostic value of this highly reproducible rule was clear from the complete separation of 27 survivors and six nonsurvivors in the learning set of 33 patients. In an independent test set, all three nonsurvivors and 13 of the 14 survivors were correctly classified, thus confirming the accuracy and reliability of the developed rule to predict the outcome of future patients with Stage I endometrial adenocarcinoma.