CA-125, a serum marker of epithelial ovarian cancer, was studied by a radioimmunometric method: the sensitivity and specificity of the assay was studied in 260 patients with non ovarian carcinomas and 120 patients with non malignant diseases. The ideal threshold value has been discussed. Levels higher than 20 UI/ml (cut-off value) have been found in 53% of cases. Sensitivity falls to 25% if the cut-off value is 65 UI/ml. The serum levels correlated well with the existence of a metastatic disease (P less than 0.001). A second assay allowed to study in 163 cases the correlation between the variations of the serum level and the clinical evolution; a good correlation was found except in case of stable disease. High levels have also been found in patients with benign diseases, most of all in cases of pneumonia and severe liver cirrhosis.