The patient was a 61-year-old man admitted with the complaints of cough, arthralgia, and swelling of the legs. A chest roentgenogram and chest computed tomographic scan revealed a giant mass in the right upper lobe. Transperitoneal lung biopsy was performed, and a diagnosis of poorly differentiated adenocarcinoma was made. Physical examination confirmed swelling of the legs and clubbing of fingers on both hands. Bone scintigrams showed marked accumulation of 99m-Tc-MDP in the long bones, bones of the hands, and patellae. These findings yielded a diagnosis of pulmonary hypertrophic osteoarthropathy associated with primary lung cancer. Although a high serum level of growth hormone was also detected, immunohistochemical analysis did not find growth hormone in the tumor itself. Chemotherapy and radiotherapy were performed but did not stop progression of the disease. The patient subsequently experienced worsening arthralgia and swelling of the legs. Steroid therapy rapidly alleviated the arthralgia and swelling, but not the clubbing of the fingers. Thereafter, the patient's serum CRP and ICTP dropped to normal levels, and the abnormal findings of bone scintigrams subsequently disappeared. The pulmonary hypertrophic osteoarthropathy was not clearly attributable to growth hormone. Steroid therapy was effective in this case. Bone scintigrams and serum CRP and ICTP may be useful indicators in the therapeutic follow-up and monitoring of patients with pulmonary hypertrophic osteoarthropathy.