The CA 195 levels in ovarian cyst fluids from malignant mucinous tumours (median 2,300,000 U/ml) were significantly higher than the levels in benign mucinous tumours and malignant non-mucinous tumours (medians of 26,800 and 1,700 U/ml, respectively, p = 0.039 and 0.011). Also, the carcinoembryonic antigen (CEA) and tumour-associated trypsin inhibitor (TATI) levels in cyst fluid from mucinous tumours were much higher than those in non-mucinous tumours. Pre-treatment serum CA 195 levels were found to be elevated ( > 10.5 U/ml) in 72% of the patients with malignant mucinous tumours (n = 25), compared with 35% in malignant non-mucinous tumours and 28% in benign mucinous tumours. The positivity rate shown for serum CA 195 in malignant mucinous tumours was higher than that measured for serum TATI, CA 125 or CEA (60, 53 and 42%, respectively). Measurement of serum CA 195 can be valuable in the clinical management of patients with mucinous ovarian cancer.