Neutrophils are believed to play an essential role in the pathogenesis of the adult respiratory distress syndrome (ARDS). This concept is largely based on the observation that neutrophil depletion protects against altered pulmonary vascular permeability in several models of acute lung injury produced in laboratory animals. Four patients who developed ARDS during periods of profound neutropenia are presented. These patients met commonly accepted clinical and roentgeno-graphic criteria for the syndrome, and each had the characteristic findings of diffuse alveolar damage by lung histologic examination. The failure of this degree of neutropenia to protect against ARDS in humans raises questions about whether neutrophils or neutrophil products are essential in the pathogenesis of the syndrome.