CA 125 was measured during early chemotherapy in 121 patients with FIGO stage III or IV ovarian cancer to investigate if the antigen could be used as a prognostic parameter. CA 125 was determined before the start of chemotherapy and 1 month after the first, second, and third course. The antigen level before the start of chemotherapy held no prognostic information. CA 125 was a significant prognostic parameter in all three courses but its correlation with survival improved with the number of courses. Patients with high marker levels (greater than 100 U/ml) 1 month after the third course had a median survival of 7 months. This should be compared with a 50% 5-year survival in patients who had 10 U/ml or less and a median survival of 22 months among patients with intermediate CA 125 levels. Cox regression analysis of the covariation between survival, CA 125, and five variables (age, FIGO stage, histopathology, tumor grade, and bulk of residual tumor) showed that the CA 125 value was the most significant prognostic parameter. As a consequence of this study, chemotherapy of patients with high CA 125 levels 1 month after the third course may be discontinued and replaced by palliative therapy if other curative regimens are not available.