Diffuse pulmonary ossification is a rare condition of unknown pathogenesis in which mature bone is found in the pulmonary parenchyma. It is almost invariably discovered as an incidental finding at autopsy. Most commonly, it affects middle-aged men and is asymptomatic. We present the case of a 75-year-old man in which the chest X-ray showed diffuse interstitial infiltrate. Diagnosis was based on histopathological examination by open-lung biopsy, which revealed interstitial fibrosis with pulmonary ossification.