Forty-five patients with severe blunt injuries were examined during eight posttraumatic days. At random, 14 patients were given three doses of methylprednisolone intravenously; 10 mg/kg at 8-hour intervals. Fat embolism syndrome was diagnosed in 13/45 patients, only one of whom had received corticosteroid (p = 0.03). Shock, acidosis and elevated plasma catecholamines showed no correlation with the occurrence of fat embolism syndrome. Platelet counts immediately after trauma were significantly lower in the fat embolism patients than in the other trauma patients, indicative of early platelet aggregation. Prophylactically administered methylprednisolone in pharmacological doses appeared to inhibit the emergence of fat embolism syndrome.