A 72-year-old man was referred to our hospital with complaints of cough, facial rash, proximal muscle pain and weakness. Chest computed tomography (CT) revealed a nodule in the right S6, interstitial pneumonia in bilateral lower lobes and mediastinal lymph node swelling. A biopsy specimen of the nodule revealed non-small cell lung carcinoma. Gottron's sign was noted on his hands, and elevated skeletal muscle enzymes were recognized. Based on clinical and histopathological examinations, the patient was given a diagnosis of dermatomyositis. He was treated with chemotherapy (carboplatin/paclitaxel) for lung cancer and his dermatomyositis was treated with steroids (1 mg/kg of prednisolone) for prolonged muscle pain and cough. Although both therapies were successful, he died of respiratory failure due to acute exacerbation of interstitial pneumonia. In the present case, we found that decreasing tumor size might be related to the activity level of skin and muscle symptoms, not interstitial pneumonia. A combination of 3 diseases is thought to be very rare, and we discussed the intercorrelation among lung cancer, dermatomyositis and interstitial pneumonia with a review of the literature.