A 61-year-old woman had been coughing up blood-tinged sputum since May 1998. Chest radiography and computed tomography (CT) scans revealed a solitary mass (3 cm in greatest dimension) in the right lower field, accompanied by a surrounding area of ground glass and reticular appearance. Surgical lung biopsy was performed to the surrounding area. The pathological diagnosis was pulmonary ossification of the dendriform type. Alveolar macrophages obtained from her lung differentiated into tartrate-resistant acid phosphatase (TRAP)-positive multinucleated giant cells (MGCs) in the presence of autologous T cells or of macrophage colony stimulating factor (M-CSF) and interleukin-4 (IL-4). This results suggest the possibility that monocytes/macrophages may have the ability to form osteoclasts in the presence of cytokines that may be involved in the development of pulmonary ossification.