The clinical value of the tumor marker human chorionic gonadotropin-beta (hCG-β) in ascitic fluid for the differentiation of malignancy-related and nonmalignant ascites was evaluated. Ascitic fluid protein, cholesterol and cytological examination were determined for comparison. Thirty-six patients with malignancy-related ascites (27 peritoneal carcinomatosis, 9 miscellaneous malignant causes without peritoneal carcinomatosis) and 69 patients with nonmalignant ascites (55 with liver cirrhosis, 14 with miscellaneous nonmalignant causes) were investigated. hCG-β concentrations were elevated in malignant samples and with a cut-off value of 10 mlU/ml hCG-β yielded a sensitivity of 61 %, specificity of 94% and efficiency of 83%. Ascitic fluid protein (cutoff value 3.0 g/l00 ml) and cholesterol (cut-off value 45 mg/l00 ml) concentrations showed a sensitivity of 64%/83%, specificity of 77%/81% and efficiency of 72%/82%. The combination of hCG-β and cytological examination yielded 89.5% differential diagnostic efficiency, superior to the combinations of protein and cytology or protein and hCG-β. hCG-β tended to be superior to protein/cholesterol determination regarding sensitivity (44% vs. 50%/33%) and specificity (79% vs. 50%/57%) in the subgroups of patients with miscellaneous causes of ascites. In conclusion, hCG-β is frequently elevated in malignancy-related ascites and seems to be as useful a parameter as total protein for the differentiation of malignancy-related from nonmalignant a