Transbronchial biopsy (TBB) has been considered to be inadequate for the diagnosis of bronchiolitis obliterans organizing pneumonia (BOOP). We describe herein two patients with interstitial pulmonary disease in whom the diagnosis of BOOP was achieved by TBB. The two patients presented with progressive dyspnea, cough, tachypnea, and fine end-inspiratory crackles. The radiologic findings disclosed patchy alveolar infiltrates. Pulmonary function tests showed a restrictive pattern and decreased diffusing capacity. The pathologic findings disclosed bronchioles, alveolar ducts, and alveoli infiltrated with mononuclear cells. The lumina were obliterated with fibroblasts and loose granulation tissue. Corticosteroid treatment resulted in significant improvement. Transbronchial biopsy should be considered as a useful diagnostic tool for BOOP.