The notion that all children need to achieve adaquate levels of physical fitness would find few dissenters. However, the best means to achieve this important national goal is the subject of considerable debate among scholars and decision makers. Recently, concern has again shifted to school physical education programs and the time/budget/space constraints imposed on them. Given these constraints, questions arise as to the efficacy of PE to influence children's fitness. Therefore, insights on the factors that influence fitness over time could lead to clearer priorities and more efficient program planning. The purpose of this study was to examine growth rates (i.e., change) in fitness performance by children in grades 4-8 during two consecutive school years. The basic design of this study was multicohort (i.e., grades 4-8, N=91) with six repeated measurements taken over 21 months. We used the FITNESSGRAM to test each child's aerobic capacity, muscular endurance, and flexibility at the beginning (September) midpoint (February) and end (May) of two school years (2004-05, 2005-06). (NOTE: Children received 2 PE classes/week.) We used hierarchical linear modeling (HLM) to analyze the data. HLM is a special type of regression that models intercept and slope for each participant and permits hypothesis testing about rate of change over time, as well as factors associated with change. In addition to growth rates in each fitness parameter, we examined the association of anticipated predictor variables including age, sex, BMI, and sports participation. Our results varied considerably across the three areas of fitness, but four principal findings emerged: a) Very substantial gains occurrred in aerobic capacity (i.e., PACER score) for the entire sample (&Gamma=.67, t(90)=11.003, p<.001). On average, each child improved their PACER score by .67/month or by 14 laps over 21 months. b) We found significant (p<.05) but minimal gains in muscular endurance (i.e., push ups and curl ups) and flexibility; c) high levels of BMI exerted an negative pull (p<.05) on aerobic capacity (-1.09 ), push ups (-.30), and curl ups (-.90); and d) participation in after school sports was positively (p<.05) associated with higher PACER scores; that is, children in sports exceeded nonparticipants by .33 laps per month or 7 laps over 21 months. These results suggest that PE --especially in combination with sports--might foster aerobic capacity among children in grades 4-8. However, meaningful gains in flexibility and muscular endurance may require specific, targeted programs or interventions.