CA 125 was measured in peritoneal fluid from 200 patients with primary ovarian malignancies (35) and benign gynecologic conditions (165). In 86 patients CA 125 was measured both in peritoneal fluid and in serum. Patients with ovarian cancer had markedly greater serum CA 125 levels compared to patients with benign disease. CA 125 levels in peritoneal fluid were usually higher than serum levels. Twenty-six (93%) of 28 patients with ovarian cancer had peritoneal fluid levels which exceeded serum levels in paired samples. peritoneal fluid CA 125 values greater than 200 U/ml identified ovarian cancer patients with 96% sensitivity and 99% specificity. Serum CA 125 values greater than 35 U/ml identified ovarian cancer patients bearing ascites with a sensitivity of 99% and specificity of 94%. Only 2 of 165 patients with benign gynecological conditions had peritoneal fluid values above 200 U/ml. By contrast, only two values below 200 U/ml were found in ascitic fluids from 35 patients with ovarian cancer. CA 125 levels in peritoneal fluid deserve further evaluation for follow-up of patients with ovarian cancer.