Diffuse pulmonary ossification is a rare entity that presents with the formation of mature bone in the pulmonary parenchyma and is associated with diffuse and chronic lung disease, heart disease, or other system disorders. Diffuse pulmonary ossification is usually a postmortem finding by the pathologist. In the case we report, the diagnosis was established by open lung biopsy. The patient was a 79-year-old man with dyspnea, dry cough, and weight loss. He had been a smoker. A chest x-ray revealed reticulonodular bilateral pulmonary infiltrates. Computed tomography revealed interstitial disease predominantly in the septum with multiple cavitations that tended to form honeycomb patterns. Pleural thickening, retraction of the parenchyma, and bilateral fibrosis were also visible. A clinical diagnosis of interstitial fibrosis was established and the patient s course was unfavorable. An open lung biopsy was performed. The lung tissue specimens revealed zones with collapsed alveoli and others with emphysema, some of which produced secretion and erythrocytic extravasation. Interstitial vascular congestion was apparent; bronchioles presented mononuclear and some polymorphonuclear inflammatory infiltrates. Noteworthy was the presence of predominantly interstitial, multicentric foci of osseous trabeculae --some of which included adipose bone marrow. Diffuse pulmonary ossification is usually an incidental finding in autopsies of patients with a history of diffuse chronic pulmonary disease, but it is an unusual diagnosis in living patients. Diffuse pulmonary ossification is of no prognostic significance in pulmonary fibrosis. It is a marker of the chronicity and/or severity of the fibrosis.