Recent studies have shown that an influx of polymorphonuclear neutrophils into distal air spaces occurs early in the adult respiratory distress syndrome (ARDS). To study the mechanism of cell accumulation in this syndrome, we have evaluated the chemotactic activity in bronchoalveolar lavage fluid from patients with ARDS. Lavage fluid was obtained from 16 patients with ARDS within 24 h of endotracheal intubation and from 5 normal nonsmoking subjects. Lavage fluid from patients with ARDS had an average of 85% neutrophils on differential counts of cytologic preparations compared to less than 3% neutrophils for the control subjects. After removal of cells and lipids, lavage fractions of molecular weight greater than 10,000 daltons were chemotactic for human neutrophils in 14 of 16 patients, but no activity was seen with the normal lavages. Preliminary studies to identify the chemotactic factor were performed.