To evaluate the disruption of the basement membrane in the pathogenesis of adult respiratory distress syndrome (ARDS), we measured the level of 7S collagen in the bronchoalveolar lavage (BAL) fluid of patients with ARDS and normal control subjects by using a radioimmunoassay method. Twelve of 14 patients with ARDS had evidence of 7S collagen in the BAL fluid, but no 7S collagen was detected in the BAL fluid of the controls. The mean 7S collagen concentration of the BAL fluid in patients with ARDS was 19.6 +/- 9.4 ng/ml. In patients with ARDS, the BAL levels of 7S collagen correlated strongly with neutrophil counts (r = 0.813; p less than 0.01) and with elastase-complex (r = 0.914; p less than 0.01). We conclude that 7S collagen may be an index of leukocyte-mediated lung injury in patients with ARDS, and detection of 7S collagen in BAL fluid is useful for evaluating the disruption of basement membrane.