The patient was a 39-year-old man who had initially consulted our hospital at the age of 27 for an abnormal radiological finding of the chest that had been detected at a medical checkup. At that time, he had no symptoms, although a chest CT revealed diffuse reticulonodular shadows bilaterally. Video-assisted thoracic surgery was performed, and lung biopsy specimens were obtained. Histologically, there were multiple coral-like dendritic foci of bone in the lung: the larger foci occasionally contained hematopoietic marrow. The patient was diagnosed as having idiopathic dendriform pulmonary ossification. Follow-up examination after 12 years showed no significant deterioration of clinical, radiological, or physiological findings. Although idiopathic dendriform pulmonary ossification is usually diagnosed on postmortem examination, this case was diagnosed by biopsy in a patient. This is the first report of idiopathic dendriform pulmonary ossification being observed in Japan over a period of such length.