Background: Research is emerging suggesting that fitness not only improves health, but enhances academic achievement in children. Many studies have found the strongest correlation with academic achievement to be aerobic fitness. The purpose of this study is to examine the influence of aerobic fitness and academic ranking on the association between improvements in students' aerobic fitness and their academic achievement. Methods: Data were collected from 1152 second- through fifth-grade students enrolled in 10 Midwestern schools. School-fixed effects models were used to estimate the impact of improved aerobic fitness from the fall to the spring semester on students' spring percentile rankings in math and reading. Results: Students whose progressive aerobic cardiovascular endurance run improved from the fall to spring semester moved up the national spring math percentile rankings by 2.71 percentiles (p < 0.001) for all students, 4.77 (p < 0.001) for less-fit students, and 3.53 (p < 0.05) for lower performing math students. No statistically significant relationship was found between improved aerobic fitness and reading achievement. Conclusions: Improving fitness could potentially have the greatest academic benefit for those elementary students who need it the most--the less fit and the lower academic performers.