Serum levels of carcinoembryonic antigen (CEA), CA 125 and CA 15-3 were measured in 47 patients with endometrial cancer and 20 with endometrial hyperplasia. Before treatment elevated serum levels of CEA, CA 125 and CA 15-3 were found in 14, 43 and 32% of cancer patients, respectively. In the 20 patients with endometrial hyperplasia, CEA and CA 15-3 values were normal, while elevated CA-125 titers were detected in 1 case. There was an increasing incidence of abnormal levels of all markers in relation to a higher tumor stage (stage I: 36%; II: 66%; III: 100%). The percentage of positive marker values increased in case of deep myometrial infiltration (greater than M1) and/or poorer tumor differentiation (greater than G1). However, only percent increase in CA 15-3 positivity was significantly higher in more infiltrating (greater than M1, 7 vs. 65%, p less than 0.01) and less differentiated (greater than G1, 0 vs. 47%, p = 0.01) tumors. These findings suggest that CA 15-3 is in some way associated with the prognostic factors of the disease. All patients except 2 with no evidence of disease after surgery had normal serum marker values. Moreover, CA 125 and CA 15-3 levels reflected the clinical course of the disease during chemotherapy and seemed to be useful for monitoring response to treatment.