We pursued previous studies on acute and later effects of different forms of humidification on rabbits' lungs during respiratory support. Arterial wall thickening and alveolar membrane damage was confirmed in the presence of water in the inspired gas. Interstitial collagen was significantly increased 2 weeks later. Though independent of the kind of respiratory support the findings were more pronounced with positive pressure ventilation than with a low continuous pressure with spontaneous breathing. These effects did not occur after humidification with precautions against condensation, and were thus attributable to particulate water. They were interpreted as evidence for lung injury and subsequent repair from this cause. Increased width of small artery walls, possibly from vasoconstriction, was similarly related.