Serum levels of CA 15-3, an antigen expressed by human breast carcinoma cells, were measured in 58 ovarian cancer patients, 24 patients with benign ovarian tumor and 62 patients with benign gynecological conditions. Abnormally high serum CA 15-3 levels were found in 41 of 58 patients (71 %) with ovarian cancer. The percentage of CA 15-3 positivity correlated with tumor stage. Elevated serum CA 15-3 levels were found in 5 of 24 patients (20%) with benign ovarian tumor. None of the patients with benign gynecological conditions had increased serum CA 15-3 levels. Serial measurements showed CA 15-3 concentrations to be correlated with the clinical behavior of disease in 21 of 25 patients (84%). CA 15-3 serum levels were also found to be correlated with findings on second look. These results suggest that CA 15-3 may represent a useful tumor marker in ovarian cancer.