Recent physiologic studies have indirectly suggested that interstitial pulmonary edema may develop at maximal exercise in normal humans at sea level. Therefore, we compared chest radiographs taken before and immediately after incremental exercise to maximum in 5 healthy young subjects. We looked for evidence of redistribution of pulmonary blood flow, pulmonary venous distension, loss of sharp definition of pulmonary vascular markings, hilar blurring, Kerley's A, B, or C lines, peribronchial or perivascular cuffing, widening of fissures, pleural effusion, and diffuse opacity. We also quantitated radiographic density in 6 areas of the film in each subject. There was no radiographic change to suggest an increase in lung water in any lung zone in any of the subjects. Given the documented sensitivity of chest radiography in this respect, we conclude that any increase in extravascular lung water during exercise must be trivial.