In ovarian cancer patients a 6 kDa polypeptide, the tumour-associated trypsin inhibitor (TATI), can occur at elevated concentrations in both urine and serum. In this study pretreatment serum levels of TATI (cut-off point 21 ng ml-1) and CA 125 (cut-off points 35 U ml-1 and 65 U ml-1) were determined in 152 patients with epithelial ovarian cancer (115 primary and 37 recurrent) and in 267 women with benign pelvic diseases. The data obtained were correlated with the tumor stage, histological type and tumour grade. Overall, TATI showed a sensitivity of 64% and a specificity of 75%. The sensitivity and specificity of CA 125 > 35 U ml-1 were both 80%. Corresponding values for CA 125 > 65 U ml-1 were 70% and 87%. The combination of the two markers increased the sensitivity to 91% (CA 125 > 35 U ml-1) and 86% (CA 125 > 65 U ml-1), while the specificity dropped to 61% and 68% respectively. TATI was clearly superior in mucinous carcinomas of the ovary, the rate of true-positive findings in these neoplasms was 67% vs 42% for CA 125 > 35 U ml-1 and 33% for CA 125 > 65 U ml-1. Unlike CA 125, TATI correlated well with tumour grade. The combination of the two markers had a higher negative predictive value, i.e. 93% (CA 125 > 35 U ml-1) and 90% (CA 125 > 65 U ml-1) respectively. It is concluded that, while TATI cannot replace CA 125 in the diagnosis of malignant epithelial carcinomas of the ovaries, it is a valuable additional marker in cases of mucinous carcinomas and in combination with CA 125.