We are yet to understand how continuous participation in organized sports, dropout from organized sports, or complete non-participation affect adolescents' trajectories of physical fitness and body mass index (BMI). Thus, the aim was to examine longitudinal changes in cardiorespiratory and muscular fitness, and BMI between adolescents 1) who continued or started organized sport participation, 2) who dropped out, and 3) who never participated in organized sport or dropped out before adolescence.Longitudinal observational study.Over four years (2017-2021), sport participation, cardiorespiratory and muscular fitness, and BMI data were collected annually from 963 participants (Mage = 11.25 ± 0.31). Latent growth curve models were utilized to examine levels (baseline) and slopes (rate of change) of BMI, cardiorespiratory, and muscular fitness in each sport participation group.Fitness levels significantly varied among groups. Continuing sport participants exhibited the highest levels, non-participants the lowest. Both groups showed significant improvements in cardiorespiratory and muscular fitness over time. Dropouts had higher baseline fitness than non-participants but demonstrated no change in cardiorespiratory fitness over time and a significantly smaller increase in muscular fitness than the two other groups. BMI increased similarly in all groups, with non-participants starting at higher baseline levels.Individuals who continually participated in sports maintained higher levels of fitness than individuals who did not participate in organized sports across adolescence. However, individuals who dropped out of organized sports, showed plateau in their fitness improvements, suggesting that the physical activity previously obtained through organized sports may not be replaced elsewhere.