We studied four patients with idiopathic Bronchiolitis Obliterans Organizing Pneumonia (BOOP) diagnosed by open lung biopsy. All of four presented with acute onset and a rapid course with progressive dyspnea. Their chest X-ray films showed ground-glass opacities with reduced volumes in the bilateral lower lobes. Their chest CT revealed marked increases in lung density with air bronchograms. Radiologically, it was difficult to distinguish these cases from patients with acute interstitial pneumonia. Histologically, in the uninflated specimens, the collapse of alveoli and alveolar spaces filled with foamy macrophages and proteinous exudates were observed. The organizing tissues were present predominantly in the alveolar ducts, but varied in extent among cases. We considered the reduced lung volumes on chest X-ray films and the marked increases in lung density on chest CT to reflect alveolar collapse. Proliferation of type II pneumocytes was remarkable in all cases, but reactivity to antisurfactant protein-A antibody staining was decreased in the collapsed alveoli. It was suggested that the alveolar collapse was caused, in part, by decreased pulmonary surfactant in addition to obstruction of the alveolar ducts. The rapid responses to of these patients corticosteroid therapy may be due to improvement of the alveolar collapse.