A prospective cohort study was undertaken to determine the effectiveness of air transport for major trauma patients when transferred to a trauma center from a rural Emergency Department. The null hypothesis evaluated was that there was no difference in outcome for patients transported by helicopter EMS (HEMS) when compared to patients transported by conventional ground EMS. The dependent variable of outcome was studied using the TRISS method in a group of 872 consecutive trauma patients admitted after long-distance transfer. Of this group, 110 patients transported by ground and 103 patients transported by air met the inclusion criteria. The ground EMS group had a mean TS of 14.4, a mean ISS of 25.2, and a mean Ps of 0.867. TRISS predicted 15 deaths and there were 15 actual deaths. The HEMS group had a mean TS of 11.4, a mean ISS of 34.9, and a mean Ps of 0.587. There were 46 predicted deaths and 33 actual deaths. This 25.4% reduction in predicted mortality was significant (Z = 3.95; p less than 0.001). Stratification of patients into Probability of Survival (Ps) bins allowed for outcome evaluation of groups with similar levels of injury severity. The benefit of HEMS transport was seen only in the patients with a probability of survival of less than 90%. We conclude that the major trauma patients transported by HEMS had a better outcome than those transported by ground EMS. The benefit seen with HEMS was directly related to injury severity and was demonstrated only in the patients with a Ps of less than 0.90.