A 61-year-old woman was admitted to our hospital with a two week history of knee-joint pain, dry coughing and dyspnea on exertion. A chest roentgenogram revealed bilateral patchy and infiltrative shadows. Laboratory examination revealed high CK and aldolase levels. Although myositic symptoms were absent, the respiratory symptoms rapidly worsened and respiratory failure developed. An open-lung biopsy and a muscle biopsy were done. The open-lung biopsy specimen showed bronchiolitis obliterans organizing pneumonia (BOOP), but the muscle biopsy specimen revealed non-specific findings. After 4 weeks of intravenous intermittent high-dose corticosteroid therapy, 50mg of cyclophosphamide was given daily, along with 20mg of prednisolone on alternate days. There were marked clinical, physiological and roentgenographic improvements. A test for anti-Jo-1 antibody was positive, which suggested that this patients had pulmonary manifestations of polymyositis/dermatomyositis. Although BOOP can have an indolent course and a good prognosis, it may rapidly worsen and respiratory failure may develop, in which case it should be treated aggressively with a combination of intravenous high-dose corticosteroids and immunosuppressants.