A diabetic patient presented with symptoms and laboratory findings compatible with atypical pneumonia. Despite appropriate antibiotic therapy, his dyspnea, arterial oxygenation, and chest roentgenographic findings worsened. Because the patient had a history of homosexual contact, an open lung biopsy was obtained to morphologically define the tissue reaction and to search for a specific etiology. Histologic examination showed bronchiolitis obliterans but did not demonstrate a cause. Legionnaires' disease was subsequently diagnosed on the basis of a fourfold rise in indirect fluorescent antibody titer. This case report demonstrates that Legionella pneumophila may induce lung injury with bronchiolitis obliterans. Such patients may benefit from corticosteroid treatment.