Serum CA 125 (a marker of coelomic epithelial cells) and aminoterminal propeptide of type III procollagen (PIIINP; an indicator of collagen metabolism) concentrations were measured in 148 patients with endometrial carcinoma. An initial serum concentration of CA 125 was pathologic in 17% of the patients, the frequency of abnormal values being higher (P = 0.0001) in advanced (63%) than in early disease (10%). The serum PIIINP concentration was increased in 35% of the patients and more often (P less than 0.05) so in advanced (63%) than in early disease (31%). Among all the patients, at least one of the tumor markers was increased in 43% of the cases. In early disease 12 of 108 patients contracted recurrent cancer. The accuracy of the pathologic CA 125 (9%) and PIIINP (18%) concentrations in their prediction was poor. In the total material, pathologic CA 125 and PIIINP concentrations appeared simultaneously in 11 patients, of whom eight had poor prognoses. In monitoring of treatment response of 24 patients, regression was accompanied by normal or decreasing CA 125 and PIIINP values. The persistence of pathologic CA 125 and/or PIIINP concentration predicted relapse of the malignancy. In progressive disease, CA 125 and PIIINP concentrations together or separately remained at a pathologic level or increased continuously. In clinically stable endometrial carcinoma, CA 125 gave false-negative results in 71% of the determinations and PIIINP only in 12%. The current results suggest the use of CA 125 and PIIINP, simultaneously, in monitoring the clinical course of advanced endometrial carcinoma.