The PACER test (a 20m aerobic shuttle run performed to a timed cadence) has proven to be a valid and reliable assessment of aerobic capacity in children. A problem that limits the overall utility of the PACER is that many schools do not have gyms or physical education classrooms large enough to administer the test. Purpose: This study compared the utility of the standard 20m PACER test with an alternative 15m PACER protocol in 5th and 8th grade children. Methods: A total of 171 students in 5th (M, n=37; F, n=31) and 8th (M, n=60; F, n=43) grade completed a 15m and 20m PACER protocol in a counterbalanced design. Trials were conducted in consecutive weeks at the same time of day. Correlations were computed between VO2 max estimates from the two tests. Estimates of VO2 max were directly compared using a two-way (gender x grade) Repeated Measures ANOVA. Classification agreement was also evaluated based on the FITNESSGRAM criterion referenced standards. Results: The overall difference in estimated VO2 max between the two tests was 1.2 ml/kg/min, and this multivariate F test was significant, [F(1,167)=15.7, p<.001]. The difference was slightly larger for males than females so the gender by method interaction was significant [F(1,167)=4.20, p=.042]. The overall effect sizes for these differences were small (<.30) and probably not of clinical significance. The overall correlation of VO2 estimates between the tests was moderate ([underlined r]=0.76). Correlations between VO2 max estimates were slightly higher for males ([underlined r]=0.79) than for females ([underlined r]=0.67). Correlations were higher for the 8th grade children (M, [underlined r]=0.85; F, [underlined r]=0.71) than for 5th grade children(M, [underlined r]=0.61; F, [underlined r]=0.53). The classification agreement based on meeting or not meeting the FITNESSGRAM criterion referenced standards were 88% for males and 91% for females. Conclusion: Overall, the results suggest the 15m and 20m PACER protocols provide similar information about aerobic fitness in youth. The 15m PACER protocol provides a useful alternative to the 20m protocol for schools with smaller physical education facilities.