CA 125 is a tumour marker usually used in the monitoring of ovarian carcinoma patients. This study was performed to evaluate the behaviour of CA 125 in 159 patients with benign diffuse hepatic diseases who underwent thorough clinical and laboratory evaluation. Abnormal serum levels of CA 125 were found in 40.3% of the 159 patients, 70.6% of the 85 cirrhotics and 5.4% of the 74 non-cirrhotics. There was a correlation between CA 125 and numerous biochemical parameters characteristic of liver diseases, the liver failure tests being the most relevant. Ascites was a determining factor of serum CA 125 levels (98.4% of the ascites patients and only 4.1% of the non-ascitic patients had abnormal levels), and CA 125 had excellent sensitivity, specificity, efficiency, predictive values and likelihood ratios to detect ascites. In the absence of ascites, liver dysfunction played a small but significant role in the increase of CA 125. In liver disease patients with ascites the threshold required to obtain only 10% of abnormal values was more than 50-fold the basal level. These findings invalidate CA 125 as a tumour marker in these patients.