The serum-ascites albumin difference, an index of the serum-ascites oncotic pressure difference, correlates directly with the pressure gradient between the portal capillaries and the peritoneal cavity. This test was compared with the ascites total protein concentration in the separation of "transudative" and "exudative" ascites. The serum-ascites albumin difference was large in patients with transudative ascites (1.6 +/- 0.5 g/dl) and small in patients with exudative ascites (0.6 +/- 0.4 g/dl, p less than 0.001) and provided significantly better discrimination of these categories than did the ascites total protein concentration. The serum-ascites albumin difference was especially useful in the separation of cardiac ascites, which usually has a high total protein concentration, from high protein exudative ascites. The serum-ascites albumin difference did not provide perfect discrimination of any category, however; in patients with mixed causes of ascites, this difference tended to be large, resembling ordinary transudative ascites, a potential source of diagnostic error. Nevertheless, the serum-ascites albumin difference has superior discriminatory power and should replace the ascites total protein concentration in the routine diagnostic examination of ascites.