The body's response to the effects of mechanical injury, taking the form of shock during the first hours and the onset of fat embolism in the subsequent period, is substantially higher in patients with multiple or associated injuries, both as regards the severity of manifestations and prognostic risk. Also the death rate due to this sort of complication is seen rising. Two groups of injured persons with isolated (n = 33) and multiple fractures (n = 33) were used to show that dynamic follow-up of PaO2 in the blood could serve as a criterion of the risk of fat embolism development in the body and, in particular, as a prognostic criterion for the progress of fat embolism. Special point was made of findings demonstrating a time relationship between PaO2 deterioration in the early post-injury period (up to 24-36 hours) as compared with the period of 48-72 hours after the injury. PaO2 is seen dropping rapidly in injured persons showing signs of fat embolism syndrome development. The decrease can be recorded as early as the free interval phase, i.e. prior to the manifestation of the clinical signs of fat embolism. The findings of low PaO2 levels in the blood are in accordance with respiratory ventilation disturbances and impaired diffusion documented in our investigation.