Serum CA 125 levels were evaluated in 227 patients with ovarian cancer. CA 125 levels were elevated in 86% of the patients. All histologic types, including mucinous tumors, were associated with raised CA 125 levels. There was a positive correlation with tumor burden and an inverse correlation with degree of differentiation. In patients undergoing radical operation an elevated CA 125 level was a bad prognostic index. Serial CA 125 measurements were assessable in 112 patients undergoing chemotherapy. Rising or falling levels correlated with disease in 92% of the cases. The CA 125 level increased before clinical progression with a median lead time of 3 months. Only patients who showed objective response to chemotherapy had a decrease in antigen levels of greater than or equal to 30% 4 weeks after the first course of chemotherapy and a normalization of CA 125 levels 3 months after initiation of chemotherapy. Rising levels were always associated with progression. These data suggest that CA 125 may aid in early identification of nonresponders. However, a normal CA 125 level does not exclude the presence of disease.