A 67-year-old man with a four-year history of mixed connective tissue disease (MCTD) associated with interstitial pneumonia was admitted to our hospital with a complaint of dyspnea and moist cough. Because the interstitial pneumonia was exacerbated, he was given high-dose steroid treatment (pulse therapy and sequential oral treatment of PSL 50 mg/day). After treatment his general condition showed some improvement, but then he suddenly died of acute respiratory failure. Autopsy disclosed fat emboli in the lungs, kidneys, liver and myocardium. The fat embolism may have been a consequence of the steroid treatment. Fat embolism should be taken into account as one of the causes of the acute respiratory failure in collagen vascular disease patients receiving steroid treatment.