Development of fat embolism is frequent following bone trauma involving the long bones of the legs. Determination of embolic free lipids in the plasma is based on microfiltration of lipid droplets (diameter greater than 5 micron) and nephelometric measurements after dissolution and specific precipitation. Free lipid levels in normal subjects are less than 5 mg/l. This value may rise to 37 mg/l after fracture of the leg bones and always increases after centromedullary pinning, reaching its maximum value three to nine hours after the operation. There is generally a rapid return to normal levels but sometimes this occurs slowly, particularly in patients presenting symptoms of fat embolism.