['To compare the serum-ascites albumin gradient to the exudate-transudate concept in the classification of ascites.', 'Prospective collection of ascitic fluid data from patients with well-characterized causes of ascites.', 'Hepatology inpatient and outpatient ward and consult service of a large, urban hospital.', 'A total of 901 paired serum and ascitic fluid samples were collected from consecutive patients with all forms of ascites.', 'None.', 'The utility of the serum-ascites albumin gradient and the old exudate-transudate concept (as defined by ascitic fluid total protein concentration [AFTP]) were compared for their ability in discriminating the cause for ascites formation.', 'The albumin gradient correctly differentiated causes of ascites due to portal hypertension from those that were not due to portal hypertension 96.7% of the time. The AFTP, when used as defined in the old exudate-transudate concept, classified the causes of ascites correctly only 55.6% of the time. This resulted in part because the AFTP of most spontaneously infected samples (traditionally expected to be exudates) was low, and the AFTP of most cardiac ascites samples (traditionally expected to be transudates) was high.'] Is the serum-ascites albumin gradient superior to the exudate-transudate concept in the differential diagnosis of ascites?