A 60-year-old woman was suffering from acute onset and progressive respiratory distress. Her radiographic findings showed bilateral volume loss in her lower lobes and consolidation predominantly distributed in peribronchovascular areas. The biopsied specimens performed by video-assisted thoracoscopic surgery revealed prominent fibromyxoid connective tissue within the terminal respiratory bronchioles and the alveolar spaces along the airways without marked interstitial fibrosis. No relevant cause was determined, and she was diagnosed as having idiopathic BOOP. Although her clinical course was fulminant with a poor reaction to steroid therapy, simultaneous administration of cyclosporin A and corticosteroid elicited a rapid improvement. This case report presents the effectiveness of cyclosporin A in the treatment of progressive BOOP.