It has been reported that hepatoma (HCC) cells produce abnormal proteins such as erytropietin, fibrinogen, prothrombin, and, recently, antithrombin III (AT III). In a preliminary report, we reported increased AT III levels in patients bearing HCC independent of their clinical liver status. The present study was performed to assess antithrombin III levels and other serological data present in patients with cirrhosis and in patients with cirrhosis and clinical findings of neoplastic disease. In 70 well-matched patients (47 with cirrhosis and 23 with cirrhosis and proven HCC) serum total cholesterol, albumin, prothrombin, alkaline phosphatase, AFP, aminotransferases, and AT III were determined. Together with AFP and alkaline phosphatase, patients with HCC had higher values of AT III (88 +/- 7%) and total cholesterol (184 +/- 17 mg/100 ml), as compared with cirrhotic patients (AT III 56 +/- 3.6%; total cholesterol 113 +/- 5 mg/100 ml) (P less than 0.001). No difference was observed between these two groups for albumin, prothrombin, and aminotransferases. In HCC patients, AT III levels were related to the total cholesterol level (R2 = 0.317), whereas in the cirrhotic patients it correlated with the prothrombin level (R2 = 0.274). These data suggest that in HCC patients a greater rate of synthesis of AT III occurs, whereas in cirrhotic patients lower levels of AT III occur due to impaired synthesis or increased catabolism of the protein. The serial determination of AT III in cirrhotic patients as a means of detecting neoplastic transformation is suggested.