Rare cases of primary cerebral fat embolism as a postoperative complication have been reported. In addition, cerebral fat embolism occurring before operative repair without shunt lesion are more rarely reported. We report a patient with a posttraumatic cerebral fat embolism resulting in severe neurologic dysfunction without right to left shunt. A 25-year-old man was brought to the hospital immediately after a traffic accident. He sustained a right segmental femoral shaft fracture and a left tibial shaft fracture. The patient was alert with no neurological deficits. Approximately 13 hours after injury, the patient developed acute mental status deterioration and dyspnea. Magnetic resonance imaging of the brain revealed extensive multifocal infarction owing to embolic showering throughout nearly the entire brain parenchyma. Computed tomography with intravenous contrast revealed no evidence of embolism in the lung, abdomen, and pelvis. Transthoracic and transesophageal echocardiogram revealed no circulating embolic particles or intracardiac shunt. The patient underwent closed reduction and internal fixation with a reamed intramedullary nail in the tibia and underwent open reduction and internal fixation with a reamed nail in the femur at 5 days after injury. We performed decompression of a hematoma containing a large number of lipid droplets via a small incision in the femur shaft fracture, established suction drainage of the tibia medullar cavity, and applied a tourniquet to the thigh in the tibia shaft fracture. Supportive medical treatment included endotracheal ventilatory support and tracheostomy. The patient was discharged from the hospital 50 days after admission. On follow-up 2 months later, he had returned to activities of daily living, however a speech disturbance remained.