Interstitial pneumonitis may be the presenting manifestation of polymyositis-dermatomyositis, or may occur later in the evolution of the 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. The histopathologic features are those of organizing and interstitial pneumonitis and pleuritis, with variable fibrosis. In the present series, the patients with mixed alveolar and interstitial infiltrates on chest radiograph and organizing pneumonia and bronchiolitis obliterans in addition to interstitial pneumonitis. In one patient evolution from pulmonary inflammation to interstitial fibrosis was demonstrated. The etiology of primary lung disease in PM-DM is not known, but cell-mediated autoimmunity to an unidentified component of lung tissue is suggested. Including the present series, 50 percent of patients have responded favorably to corticosteroids with decreased dyspnea, clearing of the chest radiograph and improved pulmonary function tests.