A 48-year-old man with small cell lung cancer developed ARDS, and massive pulmonary edema fluid was obtained with the fiberoptic bronchoscopy. The pulmonary edema fluid to serum ratios of total protein and albumin were 0.72 and 0.85 respectively. The ratio of LDH was higher (2.71), while that of cholesterol was lower (0.11) than that of total protein. Simultaneously, isopropyl N [I-123] p iodoamphetamine (I-123 IMP) and I-131 human serum albumin (I-131 HSA) were injected into this patient. Samples of blood and pulmonary edema fluid were collected to measure the clearance through the pulmonary microvasculature. The time activity curves of I-123 IMP and I-131 HSA in his blood samples revealed almost constant radioactivity from 5 minutes to 120 minutes after injection, while both radioactivity levels in pulmonary edema fluid samples increased with time. The clearance ratio of I-123 IMP to I-131 HSA was constant at each sampling time (mean +/- SD, 1.51 +/- 0.32). The linear correlation between I-123 IMP clearance and I-131 HSA clearance (r = 0.95, p less than 0.01) suggested that the clearance ratio of exudative plasma components may remain unchanged even if pulmonary microvasculature permeability has changed.