The cause of acute dyspnea in chronic obstructive pulmonary disease is challenging. A 69-year old man complained of dyspnea, and cough. He had a diagnosis of chronic obstructive pulmonary disease for 6 years. Oxygen therapy started for respiratory failure. At the 6th day of hospitalization, dyspnea progressively increased. Chest X-ray showed a right-sided radiolucency. The preliminary diagnosis was spontaneous pneumothorax but chest computerized tomography-scan demonstrated a space-occupying lesion in left main bronchus. The latter diagnosis was mucoid impaction but bronchoscopy revealed a malignant lesion. In conclusions, lung cancer can be a rare cause of acute dyspnea.