A 70-year-old woman was admitted in April 1997 because a coin lesion in the left middle lung field was observed on chest roentgenograms. The lesion had slightly increased in size since 1994, when it was first pointed out. Chest computed tomography elucidated a high density area with irregular borders in the center of the shadow in S 3 a. Transbronchial biopsy revealed adenocarcinoma, and a left upper lobectomy was performed. The resected specimen demonstrated a tumor with central ossification and pleural indentation. Histologic examination determined that the the tumor was a papillary adenocarcinoma with central ossification containing marrow tissue. Primary lung cancer with central ossification is very rare, and only 6 cases have been reported to date, including this case.