Recent studies link acceptable levels of aerobic capacity to reduced occurrence of high blood pressure, coronary heart disease, obesity and diabetes (see Cooper,1999). While there is considerable cross-sectional evidence on children's aerobic performance, there is little information on their aerobic performance over an extended time (e.g., a school year). The purpose of this study was to examine growth rates in aerobic performance by children in grades 4-8 during a 9-month school year. The basic design for this study was multi-cohort (grades 4-8) with 5 repeated measurements. Two university professors and the school PE teacher administered the PACER subtest of the FITNESSGRAM (1999) to each child (N=114). We tested the children 5 times at 7-week intervals during the 2004-2005 school year. We used Hierarchical linear modeling (HLM) to analyze the data. While traditional regression models slope and intercept for an entire sample, HLM models slope and intercept for each participant and provides for hypothesis testing relative to possible correlates of growth patterns. In addition to changes in aerobic performance over time (the criterion variable) we examined the association of selected predictor variables (age, sex, and participation in aerobic sports--basketball, soccer, cross-country) with changes in aerobic performance. At intercept (measurement 1), children involved in aerobic sports completed over 7 laps more than their non sports participating peers (γ=7.36, t(108)=3.44, p<.001). Age was significantly associated with increased aerobic performance at intercept as each additional year of chronological age was associated with running 1.77 more laps (γ=1.77, t(108)=2.38, p=.019. Significant improvement for the entire sample occurred across the five measurement times (slope) (γ=3.84, t(108)=14.29, p<.001) with a mean increase of nearly 4 laps at each successive test occasion. Interestingly, neither age, nor sex, nor sports participation was related to increased aerobic performance during the school year (slope). In fact, we found no significant sex differences in aerobic performance either initially or across time. Our results suggest that factors other than age, sex or sports participation accounted for the steep rate of improvement that we found for the entire sample. It is possible that regularly testing children on the PACER teaches effective pacing strategies thereby substantially improving performance on this key fitness component. Further investigations should focus on the efficacy of the PACER as a teaching tool as well as an assessment tool.