401 The validity of a 20 m shuttle run test as a predictor of maximal oxygen uptake (VO2 max) was assessed using 9 male and 8 female adult subjects with varying athletic backgrounds and aerobic fitness levels. Within a 14-day period all subjects performed a multistage shuttle run test (progressive speed increase on level gradient) and a maximal treadmill test (progressive gradient increase at constant speed) in a randomized order. The VO2 max values predicted by the shuttle run test were significantly lower than those obtained on the treadmill, for male, female, and all subjects pooled together (p<0.01). When data were divided into three groups by treadmill VO2 max readings (40-50, 50-60, and 60 + ml·kg−1·min−1) the shuttle run test continued to significantly underpredict scores (p<0.01). The underprediction was of least significance in the group with treadmill VO2 max levels from 40-50 ml·kg−1·min−1. Maximum heart rates recorded for treadmill testing were considerably higher than those for the shuttle run test (p<0.001). Correlations showed a stronger relationship between directly measured and predicted VO2 max for female subjects (r=0.90) as compared to male subjects (r=0.81) and all subjects together (r=0.86). The shuttle run test remains an effective field test for the prediction of VO2 max for larger groups, however, this research indicates that it tends to underpredict VO2 max, and that this underprediction may be more extreme within certain fitness ranges.