Polymyositis (PM) is a inflammatory connective tissue disease involving predominantly skeletal muscles, characterized by symmetrical, proximal muscle weakness, inflammation, and frequently, degeneration. Interstitial lung disease in association with PM occurs in 5~10% of cases and carries an especially grave prognosis. Although the cause of lung involvement in PM is not known, the underlying pathologic process in the lung is an immune mediated inflammation of alveolar structures, alveolitis. It is of interest, therefore, that cyclophosphamide, an immune modulating agent, has been reported to be effective in the treatment of PM. We report a case of corticosteroid resistant PM associated with interstitial lung disease, successfully treated with cyclophosphamide. A 37-year-old female was presented with 8 months duration of cough, exertional dyspnea, and muscle weakness. She had typical symptoms, physical findings, and elevated muscle enzyme levels in serum with characteristic findings of muscle biopsy. She also had typical interstitial lung disease pattern on chest X-ray and high resolution CT with restrictive pattern on pulmonary function test. The findings of transbronchial lung biopsy was compatible with interstitial lung disease. She failed to respond to corticosteroid initially. Subsequently steroids and cyclophosphamide were given with excellent clinical improvement.