Complications of prehospital use of the military anti-shock trouser (MAST) in treating hypotensive and hypovolemic patients have been few and minor. Undesirable consequences--compromised respiratory excursion or autonomic disturbances such as emesis, urination or defecation--were rare. Forty-seven (88.7%) of 53 patients with conditions classed as potentially lethal or severe survived. Close communications between prehospital and emergency department personnel can assure accurate transmission of circumstances of MAST application and result in optimal responses and improved inhospital patient care. Both "false positives" (supposed initial hypotension or hypovolemia) and "false negatives" (seeming stability in the face of major anatomic derangements) require a carefully structured operational sequence. Further evaluation of the device, particularly in cardiovascular/cardiopulmonary cases, is in progress.