Elevated levels of CA 125 have been shown to be present in the serum of patients with ovarian carcinoma, non-gynaecological cancers and some benign diseases. However, the value of CA 125 in malignant peritoneal mesothelioma has not been studied in detail. Therefore, 7 patients with diffuse malignant mesothelioma were included in this study. Median age was 52.4 (range 16-73), and 6 of them were women. The mean serum CA 125 level was 308 kU/l (range 8-1,300 kU/l). Serum CA 125 concentrations were found to be elevated in all 6 female patients. In 3 patients, serum CA 125 levels were followed prospectively and showed a very close correlation with the response to chemotherapy. In 2 responder patients, initially elevated CA 125 levels returned to normal during remission after chemotherapy. In 1 non-responder patient, the serum CA 125 level continued to rise. In conclusion, the serum CA 125 level might be helpful in the diagnosis and follow-up of malignant peritoneal mesothelioma.