In a study of 33 newly diagnosed patients with ovarian cancer, we have correlated serum levels of the tumour marker CA 125 with disease stage, response to cytotoxic therapy and survival. Elevated levels were found in 22/28 patients (79%) with Stage III/IV disease, compared with 0/5 patients with Stage I disease and 7/38 patients (19%) with non-ovarian malignancies. Of 15 patients with measurable disease, serum CA 125 levels correlated with clinical response to treatment in 11; the remaining 4 patients had persistently elevated CA 125 levels in spite of clinical response and subsequently developed early relapse. Among the 13 patients with no measurable disease, 11 remained in clinical and radiological remission with treatment, with a fall to normal CA 125 levels; 2 patients who had persistently elevated CA 125 levels developed disease recurrence after completion of chemotherapy. Sequential measurements of CA 125 for up to 2 years in 8 patients showed that rising CA 125 levels preceded clinical relapse by a median of 3 months. The most important prognostic value of CA 125 was the level following 3 cycles of chemotherapy, independent of clinical status--median survival for 15 patients with normal CA 125 at 3 months was 15+ months compared with a median survival of 6 months in 13 patients who had elevated CA 125 at 3 months. These data confirm that 1) serum CA 125 levels are more sensitive than clinical or radiological assessment in monitoring response/relapse in ovarian cancer and 2) the CA 125 level after 3 cycles of chemotherapy is of major prognostic value in predicting survival.