Various biochemical indexes discriminate neoplastic from nonneoplastic ascites. However, within the latter group, the distinction between cirrhotic ascites and ascites caused by hepatocarcinoma (HC) is usually based on liver biopsy or cytology. HC-derived ascites is included in the group of nonneoplastic ascites because it is not associated with peritoneal spreading of neoplastic cells. In 54 cases of cirrhotic ascites and 17 cases of HC ascites, all histologically diagnosed, ascitic pseudouridine concentrations discriminated cirrhotic from HC ascites. For example, using the cutoff value of 4.25 mumol/L (obtained by ROC curve analysis) resulted in a diagnostic sensitivity of 88.2% and a diagnostic specificity of 90.8%. Moreover, in cirrhosis, the ascitic concentrations of pseudouridine were lower than serum concentrations, and the two sets of values were correlated; in HC, however, ascitic pseudouridine concentrations were higher than serum concentrations, and the two were unrelated. These findings strongly suggest that in cirrhotic patients ascitic pseudouridine derives from serum by diffusion, whereas in HC patients the mechanism appears to be more complex.