Background: Health-related fitness knowledge (HRFK) has been an essential concept for many health and physical education programs.There has been limited understanding and longitudinal investigation on HRFK growth.This longitudinal study examined HRFK growth and its individual-and school-level correlates in middle school years under 1 curriculum condition: Five for Life.Methods: Participants were 12,044 students from 47 middle schools.Data were collected at both individual/participant and school/institution levels.Individual-level variables included gender,grade,and HRFK test scores.School-level variables included percentage of students receiving free and reduced meals (FARM),student-to-faculty ratio for physical education,and school academic performance (SAP).We used hierarchical linear modeling to examine HRFK 3-year growth in relation to individual-and school-level correlates.Results: The average HRFK score at 6th grade for females was 42.81％ ± 1.32％.The predicted HRFK growth was 17.06％ ± 1.02％ per year,holding other factors constant.A 1-standard deviation increase in FARM correlated with a 14.68％-point decrease in predicted test score (p =0.02).A 1-standard deviation increase in SAP was associated with an 11.90％-point increase in HRFK score.Males had a significantly lower growth rate than females during the middle school years (0.78％/year,p =0.02).Conclusion: The result showed that both individual-and school-level variables such as gender,FARM,and SAP influenced HRFK growth.Educators should heed gender differences in growth curves and recognize the correlates of school-level variables.