The prognostic significance of serum CA 125 level before treatment of relapse for invasive epithelial ovarian cancer has been evaluated in 135 patients. At time of relapse, serum CA 125 level was higher than 35 U/ml in 110 (82%) patients. Those with serum CA 125 level of 35 U/ml or less at relapse had a better prognosis than those with higher values. Among patients with serum CA 125 level higher than 35 U/ml no difference in survival was observed. By multivariate analysis the independent prognostic factors for survival were histologic type (P < 0.0001) and serum CA 125 level (P < 0.01). Randomized trials are needed to further evaluate whether early detection of disease relapse because of serial serum CA 125 measurements does have prognostic significance.