Pulmonary hypertrophic osteoarthropathy (PHO) is a syndrome of unknown etiology, presenting with osteitis of the long bones, arthritis, and clubbing of digits, associated with lung cancer or other chronic pulmonary or pleural diseases. Although about 3-10 percent of cases of lung cancer were reported to be associated with PHO in USA or Europe, reports of these cases are relatively rare in Japan. We report two cases of PHO associated with primary lung cancer, in which symptoms were rapidly improved by resection of the primary lesions. Both cases were poorly differentiated adenocarcinoma of the lung (pT2N0M0, Stage I). The bone and joint pain rapidly improved soon after the resection of the primary lesions. The abnormal findings of bone scan and bone X-ray film subsequently improved. In the 2nd case, radionuclide imaging more clearly revealed the presence and extent of subperiosteal activity than radiography. Radionuclide disclosure of PHO was considerably more sensitive than radiography.