A 64-year-old woman presented with exertional dyspnea. The case was diagnosed as mixed connective tissue disease (MCTD) due to presence of swollen fingers, Raynaud's phenomenon, muscle weakness, positive anti-U1RNP antibody, pericarditis and interstitial pneumonia. Although the histology from a transbronchial lung biopsy (TBLB) indicated organizing pneumonia, corticosteroid therapy was postponed for two months at the patient's request. She died 8 weeks later from acute progressive interstitial pneumonia in spite of the administration of intravenous cyclophosphamide combined with prednisolone. The autopsy revealed exudative and organizing diffuse alveolar damage (DAD). Previous reports have shown that DAD is an extremely rare pulmonary complication in MCTD. This report presents a case of MCTD with acute respiratory failure. This case thus suggests that this therapy should be administered as soon as possible.