In a prospective study, there were 13 patients with cardiac ascites among a group of 262 ascites patients (5% of the total). I compared the characteristics of 20 ascitic fluid samples from these patients with heart failure to those of 20 patients with cirrhotic ascites. The serum-ascites albumin concentration gradient was greater than or equal to 1.1 g/dl in all patients in both groups. The ascitic fluid total protein concentration was greater than or equal to 2.5 g/dl in all patients with cardiac ascites whereas only 10% of patients with cirrhotic ascites had such high values. The ascitic fluid lactate dehydrogenase and red cell counts were significantly higher in cardiac ascites than in cirrhotic ascites--although cardiac ascites was not visibly bloody. The peripheral hematocrit of patients with cardiac ascites was also significantly higher than that of patients with cirrhotic ascites. The ascitic fluid analysis in patients with cardiac ascites is characteristic and may assist in the differential diagnosis of ascites.