To evaluate the diagnostic usefulness of radioisotope bone scan and clinical activity of myositis, bone scan using $^{99m}Tc-Methylene$ diphosphonate was serially done before and after treatment with prednisolone in 10 patients with well-documented dermatomyositis-polymyositis. The observed results were as follows. 1. In all 10 patients before treatment with prednisolone, the bone scans showed evidence of increased muscle uptake. Muscle uptake was markedly increased in 4 patients, moderately increased in 3 patients and minimally increased in 3 patients. 2. The site of increased muscle uptake was consistent with the site of clinically involved muscle which was weak and tender. 3. The degree of muscle uptake correlated with the severity of the muscle weakness and tenderness at the scan was done. In all 10 patients treated with high dose prednisolone, muscle uptake was decreased following therapy. Above results suggest the radioisotope bone scanning may be useful in the diagnosis and treatment of patient with dermatomyositis-polymyositis.