Abnormal CA 15-3 antigen levels are found in the serum of most patients with advanced breast carcinoma. Elevations of this marker are less frequently seen in other malignancies. Circulating CA 15-3 levels might be useful in the differential diagnosis of the primary site of cancer. We studied the levels of CA 15-3 in 500 patients with different non-mammary cancers. Elevations of CA 15-3 (greater than 40 U ml-1) were observed in all types of epithelial malignancies, especially in ovarian (46%), respiratory (26%) and liver (30%) carcinomas. Abnormal values were observed in some patients with haematological malignancies and sarcomas, but not in melanoma or neurological tumours. CA 15-3 antigen levels correlated with the extent of non-mammary malignant tumours. Patients with locoregional cancer had a significantly smaller proportion of elevations of the antigen than those with distant metastases (12% versus 35%, P less than 0.001). In particular, elevated CA 15-3 levels were observed in 70% of patients with metastatic ovarian cancer. Liver involvement by cancer did not produce more elevations of CA 15-3 than metastases to other organs (32% versus 39%). Simultaneous determination of circulating CA 15-3 and CA 125 antigens in 58 patients with cancer of the ovary showed that CA 15-3 is elevated in some cases of ovarian carcinoma with non-elevated CA 125, and that CA 15-3 and CA 125 are distinct antigens. We conclude that circulating CA 15-3 antigen levels can be found elevated in virtually all types of cancer, particularly when distant metastases are present. Therefore, CA 15-3 levels should not be used in the differential diagnosis of the primary site in patients with metastatic malignancies of unknown origin. Evaluation of CA 15-3 levels may enhance the sensitivity of CA 125 in monitoring the course of ovarian carcinoma.