Fat embolism syndrome is a rare but serious complication occurring most of the time in patients with long bone fractures. And it occasionally occurs when patient had underlying disease. For example, pancreatitis, diabetes mellitus, alcoholic liver disease and connective tissue disease can be risk factors. The 44-year old woman visited to the Korea university hospital because of sudden dry cough, blood tinged sputum, and exertional dyspnea. We found petechiae on her anterior chest wall. Chest X-ray and CT showed patchy opacities and multifocal ground-glass opacities in both lung fields. Open lung biopsy demonstrated diffuse pulmonary hemorrhage and intravascular macrovesicular fat bubbles. After conservative management, her symptoms and radiologic findings were significantly improved. We report a case of fat embolism syndrome without any known risk factors.