We investigated the lung injury that occurs following reexpansion of a unilateral pneumothorax and determined the effect of superoxide dismutase (SOD) infused immediately prior to reexpansion on this injury. After 7 days of at least 80% right pneumothorax, rabbits received intravenous infusions of either SOD (n = 7), heat-inactivated SOD (n = 1), or vehicle (n = 7) immediately before lung reexpansion. Lung injury was assessed by measuring the systemic white cell counts, pulmonary blood volumes, extravascular albumin, extravascular lung water, wet/dry weight ratios, and histology 2 h after reexpansion. The reexpanded lung showed increased extravascular albumin, extravascular lung water and wet/dry weight ratios with decreased blood volumes compared to the uninjured lung. SOD delayed the onset of leukopenia and neutropenia at 3 and 7 min after reexpansion, but the white cell counts had decreased to the same level in both groups by 30 min. SOD had no effect on the degree of injury after 2 h. While a single bolus of SOD given immediately before reexpansion delayed the onset of this injury, it did not affect the injury that subsequently developed in the lung.