A prospective study of 100 patients with isolated skeletal injuries revealed that seventy had inapparent hypoxemia(Pao2 = eighty millimeters of mercury) and twenty-four were severely hypoxic (Pao2 = seventy millimeters of mercury) as determined by arterial blood-gas analysis within thirty-six hours after injury. Patients with the highest risk of developing severe hypoxemia were those with fractures of the lower extremity. The incidence of hypoxemia was the same in patients with open and closed injuries, as well as in those who did and did not receive regional anesthesia or sedation. The hypoxemia appeared to be related to intrapulmonary shunting, possibly caused by subclinical fat emboli. Two patients had the full-blown clinical picture of fat embolism with frank pulmonary insufficiency. Serial determinations of arterial oxygen can identify patients in whom the fat embolism syndrome is likely to develop.