We assessed validity and reliability of the new 20m square shuttle run test (SST) for predicting maximal oxygen uptake (V O(2max)) and compared it with its predecessor, the 20m Multistage Shuttle Run Test (MST). In a repeated-measures randomised-block design, 74 healthy adult males performed the SST, the MST and a treadmill test (TT). To assess reliability, 40 of the total 74 volunteers were randomly-selected to perform the SST and MST twice. Unlike the SST (p>0.05), mean predicted V O(2max)(V predO(2max)) from the MST was significantly increased from that measured during the TT (p<0.05). The V predO(2max) from SST and MST correlated with TT V O(2max) at r=0.95 (p<0.001) and r=0.63 (p<0.001), respectively. Prediction error of SST was -0.3+/-3.3mlkg(-1)min(-1) with a coefficient of variation of +/-3.5%, while the equivalent values for MST were 4.2+/-7.3mlkg(-1)min(-1) and +/-7.4%. Mean test-retest V predO(2max) did not differ for both SST and MST (p>0.05), while the corresponding test-retest correlation coefficients were r=0.85 (p<0.001) and r=0.72 (p<0.001). Reliability errors in 95% limits of agreement were 0.3+/-4.8 and 0.6+/-6.8mlkg(-1)min(-1) while coefficients of variation were +/-5.2% and +/-6.8% for the SST and MST, respectively. It is concluded that SST is a more valid proxy than MST for predicting laboratory V O(2max) based on the current procedures, while both tests are sufficiently reliable in healthy male adults.