A 74-year-old woman had general fatigue and mild fever in August 2004. Her chest X-ray showed slight ground glass opacities in the upper and middle lung fields of both lungs. Though she was prescribed antibacterial drugs, the abnormal shadows on chest X-ray did not improve. The chest CT showed ground glass opacities and reticular shadows with thickened alveolar septa (crazy-paving appearance) in both lungs, and a clearly defined mass in the anterior mediastinum. She underwent thymo-thymectomy with wedge resection of the upper lobe of the left lung. Anterior mediastinum tumor was pathologically diagnosed as thymoma. Lung biopsy demonstrated alveoli filled with SP-A positive granular materials, and we diagnosed pulmonary alveolar proteinosis. About 1 month after operation, the shadows on chest CT showed improvement. We think there might be some relationship between thymoma and pulmonary alveolar proteinosis.