The +Gz tolerance of USAF aircrewmen undergoing medical evaluation has been tested at the USAF School of Aerospace Medicine since 1973. For various reasons, the test protocol can usually be carried out only once on these patients. Accurate determination of the +Gz tolerance of aircrewmen who fly high performance fighter aircraft is very important in assuring aero-medical safety, since loss of consciousness as a result of exceeding a pilot's G tolerance may result in loss of life and loss of aircraft. It is, therefore, necessary to estimate the variability associated with each profile of the test so that a more accurate assessment of +Gz tolerance can be made. Multiple repeat medical evaluation test protocols were performed on 17 centrifuge acceleration panel members. The standard deviations in the +Gz measurements for the four centrifuge profiles were GOR(1) = 0.38 Gz, ROR=0.22 Gz, GOR(2)=0.34 Gz, and GOR(S)=0.39 Gz. A statistically significant learning effect, which increases +Gz tolerance, was observed in both experienced and inexperienced subjects. Knowledge of the variability associated with each test profile will allow a more accurate definition of an individual +Gz tolerance when only a single centrifuge test protocol can be performed. In addition, possible future use of this centrifuge protocol in the selection of individuals with above- or below-average +Gz tolerance is facilitated with an accurate assessment of the variability associated with the test.