Blood gas analysis an various haematological measurements were studied prospectively in forty patients suffering from lower limb fractures. Hypoxaemia (arterial PO2 of less than 70 mmHg at sea level) was common with a mean value for all patients of 76 mmHg, despite O2 therapy, when indicated. None of the haematological findings was significant except that every patient with a white blood cell count of less than 7000 per mm3 was also hypoxaemic. Not all patients with hypoxaemia had a relative leucopaenia. A white blood count of less than 7000 per mm3 in the first 72 h following trauma may therefore be considered as an indication for oxygen therapy.