A 64-year-old woman presented skin lesions on her face, upper extremities and finger erythema (heliotropism and Gottron's sign). She had weakness in her lower extremities. She was given a diagnosis of dermatomyositis (DM), because the serum examination showed that a myositis-specific antibody was positive whereas Jo-1 antibody was negative. The findings of chest X-ray and computed tomography showed that she had limited small cell lung cancer, but no interstitial pneumonia. She was treated with standard chemotherapy consisting of cisplatin and etoposide with accelerated hyperfractionation radiotherapy. She showed partial response to the treatment, whereas the skin lesions and muscle weakness deteriorated accompanied with bone marrow suppression due to chemotherapy. Skin and muscle biopsy were performed and pathological findings showed typical perivasculitis infiltrated with lymphocytes in muscle and skin. With the recovery of bone marrow suppression and partial response due to chemotherapy, the skin lesions improved and creatine kinase became normalized. She was given a diagnosis of paraneoplastic DM. Since severe leukocytopenia paralleled the deterioration of DM, the decrease of peripheral white blood cell counts, especially regulatory T cell counts, may be associated with DM activity.