To investigate whether biochemical changes soon after injury could be used as a measure of its severity or to predict its outcome we have examined the relationships of an injury severity score, initial blood metabolite concentrations, and subsequent biochemical changes in 33 injured patients. At about 4 hours after injury severity was related directly to blood lactate, pyruvate, and alanine (p< 0.01), and inversely to blood ketone body concentrations (p< 0.05). In 19 patients the daily total nitrogen, histidine, and 3-methylhistidine excretion was measured for 7 days. Only total urinary nitrogen was related to injury severity; 3-methylhistidine was inversely related to initial ketone body concentration; and histidine excretion was related to initial lactate, pyruvate, and alanine concentration. It is concluded that certain early biochemical measurements are significantly related to the severity of injury, and thus may be of practical importance.