The proper classification of pleural effusions into transudates and exudates has great clinical significance. It is believed that the treatment of congestive heart failure may convert an associated transudative pleural effusion into a "pseudoexudate." We studied eight patients with congestive heart failure during nine episodes of decompensation with pleural effusion, which was bilateral in five and right-sided in three. Thoracocentesis was done on identification of the patient and at 6 +/- 2 days after treatment of heart failure resulting in diuresis and a mean weight loss of 5.8 +/- 3.2 kg. The mean protein level of the pleural fluid was 2.2 +/- 0.7 g/dL at the initial study and increased to 3.2 +/- 1.08 g/dL at the final study (p less than 0.01). The LDH level of the pleural fluid increased from 116 +/- 69 to 183 +/- 117 units/L (p less than 0.01). The fluid/serum ratio for protein increased from 0.34 +/- 0.09 to 0.47 +/- 0.13 (p less than 0.01) and for LDH from 0.39 +/- 0.16 to 0.64 +/- 0.28 (p less than 0.01). In three patients, pleural fluid was classified as a transudate at the initial study but met the criteria for an exudate after treatment of heart failure. Effectiveness of diuresis was measured by weight loss; a significant correlation between weight loss per day and change in the protein level of the pleural fluid was noted (r = 0.715; p less than 0.05). We conclude that the treatment of congestive heart failure causes significant changes in the pleural fluid's chemistry; in some cases, a transudate may be converted into a "pseudoexudate."