To evaluate the cellular and biochemical composition of bronchoalveolar fluid in high-altitude pulmonary edema (HAPE), we performed bronchoalveolar lavage in three climbers with HAPE in a research facility at 4400 m on Mount McKinley. Three healthy climbers were used as controls. The HAPE fluids contained marked increases in high-molecular-weight proteins, erythrocytes, and leukocytes, most of which were alveolar macrophages. The HAPE fluids also contained detectable amounts of leukotriene B4 and other lipoxygenase products of arachidonic acid metabolism, complement fragments (C5a), inhibitors of neutrophil chemotaxis, and acid proteases but not hydroxyproline, a constituent of collagen. The data from this study indicate that HAPE involves a transient "large pore" leak in the pulmonary circulation. Despite the presence of two potent mediators of inflammation, leukotriene B4 and C5a, HAPE is not characterized by the intense neutrophil accumulation that is typical of other forms of acute lung injury.