In a retrospective study 121 patients with endometrial cancer were examined. In addition, 20 primary endometrial adenocarcinomas were tested immunohistochemically for CA 125. All tumor tissues were demonstrated to contain CA 125. However, only 25% of 110 patients had elevated CA 125 levels in serum before treatment. The incidence of elevated CA 125 serum levels increased with higher tumor staging up to 55% and 86% in surgical Stages III and IV, respectively. In Stage I and II disease (International Federation of Gynecology and Obstetrics) elevated serum levels before treatment correlated with the presence of tumor tissues outside the uterine body or outside the uterus, respectively, as was determined histopathologically after operation. In addition a close correlation between elevated levels and vessel invasion of tumor cells was revealed. Serum levels of CA 125 paralleled the clinical course of disease. Tumor recurrence in the abdomen can be preceded by an increase of serum CA 125 levels.