Owing to the high false-negative rate of cytological examinations, various biochemical tests have been investigated to search for an alternative method to differentiate between the most frequent causes of ascites formation, i.e. cirrhosis of the liver and peritoneal carcinomatosis. For this purpose, serum and ascitic fluid samples of 71 ascitic patients (54 due to cirrhosis of the liver and 17 due to peritoneal carcinomatosis) were prospectively and simultaneously collected. Eleven major parameters were analyzed, and, of them, ascitic fluid protein, albumin, cholesterol, fibronectin and serum-ascitic gradient of protein and albumin were found to be useful. The serum-ascitic fluid albumin gradient was not as useful as described in the literature and, if clinically applied, the best cut-off value should be raised up to 1.5 gm/dl. Neither as useful was the serum-ascitic fluid protein gradient. Ascitic cholesterol and ascitic fibronectin were more accurate than the ascitic total protein, ascitic albumin and serum-ascitic gradient of protein and albumin in the diagnosis of peritoneal carcinomatosis. Of these tests, an ascitic fluid fibronectin concentration greater than 40 micrograms/ml was the most accurate parameter (diagnostic accuracy 96.2%) that could solely help to establish the diagnosis of peritoneal carcinomatosis.