Acute lupus pneumonitis was the presenting manifestation of systemic lupus erythematosus in six of 12 cases in this series. The clinical picture was characterized by severe dyspnea, tachypnea, fever and arterial hypoxemia. Radiographic findings included an acinar filling pattern which was invariably found in the lower lobes and was bilateral in 10 of the cases. Studies failed to reveal evidence of infection as a cause of the acute pulmonary infiltrates. All patients were treated with oxygen and corticosteroids; seven received azathioprine. Six patients survived and are clinically well 14 months to four years following their acute illness. Three of these patients have residual interstitial infiltrates with persistent pulmonary function test abnormalities indicating progression to chronic interstitial pneumonitis. Histologic sections of the lungs available from four patients revealed hyaline membranes and interstitial edema (four cases), acute alveolitis (two cases), arteriolar thrombosis (one case) and a prominent lymphocytic interstitial pneumonitis with organizing bronchiolitis (one case).