CA 125 was measured during induction chemotherapy in 119 patients with advanced epithelial ovarian cancer who had residual disease after primary surgery in order to determine whether patients with poor response to further treatment could be identified during early chemotherapy. All patients had a prechemotherapy serum CA 125 level higher than 35 U/ml. Blood samples were further obtained 4 weeks after the first, second, and third course. Four weeks after the second course of chemotherapy, all 20 patients with PCR, or microscopic disease at second-look, all 17 who achieved complete clinical remission, and 36 of 40 who achieved partial remission had serum CA 125 of 65 U/ml or less or had a decrease of 50% or more of the prechemotherapy level. Survival analysis showed that patients with a serum CA 125 level of 65 U/ml or less 4 weeks after the second course of chemotherapy had the best prognosis. In patients with a serum CA 125 level higher 65 U/ml at that time, a decrease of 50% or more of the prechemotherapy level indicated a prognosis better than that with a lesser decrease. The combined criteria for serum CA 125 (level > 65 U/ml 4 weeks after the second course and a decrease < 50% of the prechemotherapy level) allowed for identification of a real high-risk group with a median survival of 8.9 months and was identified by Cox regression multivariate analysis as the most powerful indicator for survival (P < 0.0001).