Children’s fitness, once taken for granted, is now the centerpiece of a national effort to improve the health of all Americans (e.g., Healthy People 2010). 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 (i.e., yr.1 =grades 4-7; yr. 2=grades 5-8). The study’s design was multi-cohort sequential (i.e., grades 4-8) with eight measurements taken over 21 months. We used the FITNESSGRAM to test each child’s aerobic capacity, muscular endurance, and flexibility. Hierarchical linear modeling (HLM) analyzed the data. HLM is a multi-level form of regression that models intercept and slope for each participant and permits hypothesis testing about rate of change over time, and factors associated with change. In addition to overall growth-rates on selected fitness subtests, we examined the association of age, sex, body mass index (BMI) and participation in organized sports. Our results varied considerably across the three areas of fitness, but four principal findings emerged: a) Very substantial gains occurred in aerobic performance (i.e., PACER score), (p< .001); b) we found small but significant gains in muscular endurance (pushups & curl ups) and flexibility; c) high levels of BMI (+1.5 SD) exerted a negative pull on aerobic performance, pushups and curl-ups (p<.05); and d) participation in after school sports was positively associated (p< .05) with better performance, especially PACER score. These results suggest that PE—especially in combination with sports—might foster physical fitness.