Interstitial pneumonitis may be the presenting manifestation of polymyositis-dermatomyositis (PM-DM), or may occur later in the evolution of disease. The clinical picture is characterized by non-productive cough, dyspnea and hypoxemia. The chest radiograph demonstrates interstitial infiltrates with predilection for the lung bases, often with an alveolar pattern in addition. We experienced a case of polymyositis associated with diffuse alveolar damage(DAD) that was proven in open lung biopsy. The patient was a 52 year-old woman who was presented with 6 months' duration of generalized ache, edema on ankle and wrist, non-productive cough and mild dyspnea. She had typical symptoms and physical findings of interstitial pneuminitis, and elevated muscle enzyme levels in serum with characteristic histologic findings of myositis on muscle biopsy. She also had typical interstitial lung disease pattern on high resolution CT and restrictive pattern on pulmonary function tests. The findings of open lung biopsy was compatible with diffuse alveolar damage(DAD). She failed to respond to the therapeutic trials with corticosteroid and cyclophosphamide, and finally expired due to acute respiratory distress syndrome.