To our knowledge, the effects of humidity of inspired air on bronchial blood flow in humans are unknown. During total cardiopulmonary bypass, we measured systemic to pulmonary bronchial blood flow (Qbr[s-p]) which is the volume of blood accumulating into the left side of the heart in the absence of pulmonary and coronary flow. A cannula was introduced into the right upper pulmonary vein and advanced into the lowermost portion of the left side of the heart. From this cannula Qbr(s-p) was vented by gravity and measured. Inspired gas (10 L/min, endotracheal tube, 50 percent O2 + 50 percent N2O) relative humidity was less than 20 percent and greater than 85 percent in group A (n = 25) and in group B (n = 25), respectively. Mean (+/- SE) Qbr(s-p) was 40.7 +/- 0.06 ml/min or 1.32 +/- 0.12 ml/min (percent cardiac output) in group A and 21.7 +/- 1.8 ml/min or 0.68 +/- 0.06 ml/min in group B. These data indicate that under these conditions Qbr(s-p) is increased by dry gas lung inflation in humans.