We report two patients with pain caused by malignancy mimicking postthoracotomy pain syndrome. [Case 1] A 68-year-old man who had undergone left nephrectomy because of renal cancer was referred to us for treatment of pain in the left flank. There was no pain on percussion and no pressure along the spine. However, a lumbar X-ray showed a decreased height of the 12^th thoracic vertebral body and an unclear pedicle of the 12^th thoracic vertebra. An MRI of the lumbar spine revealed mass of the 12^th thoracic vertebra spreading in the spinal canal with compression of the 12^th thoracic nerve root. [Case 2] A 65-year-old man was referred to us because of right chest pain along the scar caused by trocar insertion for a thoracoscopic lobectomy. An X-ray of the chest showed an unclear rib just beneath the painful area. A CT revealed the partially eroded 3^rd right rib, and a biopsy of it demonstrated an invasion of adenosquamous cell carcinoma. Physicians should always keep the possibility of malignancy in mind when pain is prolonged or recurrent in patients who have a history of malignancy, and even in patients who have no history of malignancy, but whose pain shows an unusual clinical course for postoperative pain.