The relationship between plasma catecholamines and injury severity was investigated in order to determine: a) whether a correlation existed between the severity of injury as assessed by the Injury Severity Score (ISS), and circulating noradrenaline and adrenaline; and b) whether such measurements were of prognostic value. It was found that in the immediate postinjury period, both noradrenaline and adrenaline correlated positively with ISS over the range of injury studied (ISS = 0–54; r = 0.67, p < 0.01). It appeared that part of the response might be attributed to psychological rather than physical factors; at lower levels of injury (ISS < 9) these psychological factors were possibly responsible for the whole of the observed effect. In the subsequent recovery period (> 24 hr postinjury) no clear relationship between plasma catecholamines and injury severity was maintained. Studies on patients who died as a consequence of their injuries failed to confirm the previously suggested prognostic value of plasma catecholamine determinations in this later postinjury period. However, plasma noradrenaline levels in this group were significantly higher on admission than in the casualties who survived serious injury (p ≤ 0.05). It is concluded that plasma catecholamine concentrations, in combination with other indices of injury severity, may provide useful information on the response to injury in man, and aid in the assessment of its overall severity in physiologic terms.