Free fatty acids (FFA) are believed to play a role in the genesis of the fat embolism syndrome (FES). Levels of plasma FFA, glucose, insulin, and the stress hormones cortisol, growth hormone, prolactin, glucagon, and catecholamines were measured on admission to hospital in: a) 14 patients with long bone fractures at risk of developing FES, b) 14 patients with soft-tissue injuries of comparable severity, and c) 20 healthy fasting subjects. The findings were similar in both groups of injured patients and in keeping with the hormonal and substrate responses to the stress of trauma; plasma FFA levels were raised but in neither group was the rise pronounced. Plasma FFA levels of the fracture patients were only moderately higher than the values of the healthy fasting subjects. These findings suggest that mobilized FFA from peripheral adipose tissue are not important in the genesis of the FES, but do not exclude a role for FFA derived mainly from hydrolysis of triglyceride emboli in the lung.