Analysis of basic pathophysiologic variables in fat embolism patients is often restricted by the complexity of the different injuries present in each individual patient. To avoid this problem we investigated the presence of the fat embolism syndrome in patients with an 'isolated' fracture of the femoral shaft. Two groups were distinguished: those who had an open fracture or a closed fracture treated operatively within 24 hours after the accident (decompression group), and those who were treated initially conservatively (nondecompressed group). Clinical fat embolism occurred only in patients in the nondecompressed group (3.5%). They showed significantly higher initial temperatures, lower pulse rates, a progressive hemoglobin decrease, and a fracture localization more proximal (p less than 0.025) than the other patients in the nondecompressed group; they also showed significantly different pathophysiologic patterns from the patients in the decompressed group. Although the pathophysiologic mechanism of the onset of clinical fat embolism remains unclear, initial temperature elevations in combination with 'typical' fracture localization and fracture type appear to have a predictive value.