A tumour-associated trypsin inhibitor (TATI) and the cancer antigen 125 (CA 125) were measured pre- or peroperatively in 30 patients with mucinous ovarian tumours (10 malignant, two borderline and 18 benign) to investigate the separate and combined use of the two markers as a diagnostic tool. In the malignant and borderline cases considered as a whole, TATI was elevated in 83% and CA 125 in 50%. The former marker was increased in one (6%) benign tumour and the latter in another (6%). The combined use of TATI and CA 125 ensured diagnosis of all malignant and borderline tumours. The specificity was 89% and the positive predictive value 86%. In conclusion, in the distinction of malignant and borderline mucinous ovarian tumours from benign ones TATI was a more reliable tumour marker than CA 125. The combined use of TATI and CA 125 ensured diagnosis of all malignant and borderline tumours in the present series.