Aerobic capacity is the ability of working muscles to consume O2 during vigorous physical activity and an indicator of cardiovascular health. This study examined change in aerobic performance by children in grades 4-8 (Age=9-14) over 14 months. One hundred and five healthy children (grades 4-8) in a small, rural, middle class school participated. All children were tested five times on the Progressive Aerobic Cardiovascular Endurance Run (PACER) at 8-9 week intervals during the 2004-05 school year, with two follow-up measurements the next September and December. Hierarchical Linear Modeling was used to analyze the data. Overall improvement in aerobic performance from September to May was statistically significant (p<.001) with children increasing their PACER scores from 26 to 49 laps. These gains were lost over the summer (p<.001), but returned to the original slope by the second follow up measurement (p<.001). Regardless of age, sex, BMI, or sports participation, children followed a similar pattern: steady improvement during the school year, loss of those gains over the summer, and return to the original slope the following year.