During an 18-month period, among 35,000 injured patients presenting to an urban trauma center, more than 3,500 were admitted, and 1,500 underwent surgery on the General Surgical Service. Three hundred fifty-two patients with prehospital systolic blood pressures of less than 90 mm Hg were randomized on an alternate day basis to receive treatment with 'MAST' (163 patients) or 'No-MAST' (189 patients). Age, mechanism of injury, prehospital management times, prehospital trauma scores, prehospital fluids administered, Injury Severity Scores, emergency center treatment, operative protocol, and calculated probability of survival were virtually identical for both groups. Eighty-eight per cent of the injuries were produced by penetrating wounds. The predominant injury area was the abdomen in 40%, the thorax in 37%, and the extremities in 13%. There was no statistically significant difference in evaluation and outcome data between the groups. Within a controlled catchment, only 2.5% of injured patients and 22% of those undergoing operation qualified for prehospital use of MAST. We conclude that for penetrating trauma with prehospital times of 30 minutes or less, MAST provide no advantage with regard to survival, length of hospital stay, or reduced hospital costs.