It is generally described that children fitness levels increase along childhood. Complementary to this idea is the notion that the tracking of children’s fitness is good to moderate during this developmental time, and that baseline (initial values) of fitness are determinant on fitness development. The importance of developmental pathways has been recently reinforced by a theoretical argument that predicts that healthy lifestyle trajectories will evolve through either a positive or negative spiral of engagement or disengagement, respectively, in various physical activity behaviors across childhood that are reciprocally linked to motor skill, perceived competence and fitness development (Stodden et al., 2008). The main goal of this study is to test the hypothesis that different developmental pathways of physical fitness do occur during childhood (6-to-9 years-old), and to test their correlation with baseline fitness status. Methods This longitudinal study design included 507 primary school children who were assessed annually for four years on seven physical fitness tests. Childhood individual trajectories (baseline and slope values) on each of the fitness tests were determined along the four years of the study. Participants were divided into three groups according to individual fitness trajectories over time: Slow Rate of Change (Slow RC Change), Average Rate of Change (Average RC), and High Rate of Change (High RC). An ANOVA 3x2 (Group x Sex) with Bonferroni post hoc tests was used to test for the differences on rate of change between the constituted groups, and sex. Correlations between slope and test values were used to analyze the possibility of fitness performance prediction along the four years of the study. Results Results showed (1)significant differences on the rate of change between all groups (p.15 for all cases);and no positive correlation between baseline values and final values on physical fitness. Discussion The fact that differential pathways of fitness development were found throughout childhood adds to the established knowledge, and can be used as fundament for children’s fitness programs. The level of fitness, even if not satisfactory early in childhood, can be changed, and it is the pathway, not the baseline, that shows to be of major importance for children to achieve a fit state. In conclusion, children show different rates of change in fitness development over childhood; and having a positive developmental trajectory of fitness predicts a fitter state at the end of childhood, independent of the initial values at 6-years-of-age.Introduction It is generally described that children fitness levels increase along childhood. Complementary to this idea is the notion that the tracking of children’s fitness is good to moderate during this developmental time, and that baseline (initial values) of fitness are determinant on fitness development. The importance of developmental pathways has been recently reinforced by a theoretical argument that predicts that healthy lifestyle trajectories will evolve through either a positive or negative spiral of engagement or disengagement, respectively, in various physical activity behaviors across childhood that are reciprocally linked to motor skill, perceived competence and fitness development (Stodden et al., 2008). The main goal of this study is to test the hypothesis that different developmental pathways of physical fitness do occur during childhood (6-to-9 years-old), and to test their correlation with baseline fitness status. Methods This longitudinal study design included 507 primary school children who were assessed annually for four years on seven physical fitness tests. Childhood individual trajectories (baseline and slope values) on each of the fitness tests were determined along the four years of the study. Participants were divided into three groups according to individual fitness trajectories over time: Slow Rate of Change (Slow RC Change), Average Rate of Change (Average RC), and High Rate of Change (High RC). An ANOVA 3x2 (Group x Sex) with Bonferroni post hoc tests was used to test for the differences on rate of change between the constituted groups, and sex. Correlations between slope and test values were used to analyze the possibility of fitness performance prediction along the four years of the study. Results Results showed (1)significant differences on the rate of change between all groups (p.15 for all cases);and no positive correlation between baseline values and final values on physical fitness. Discussion The fact that differential pathways of fitness development were found throughout childhood adds to the established knowledge, and can be used as fundament for children’s fitness programs. The level of fitness, even if not satisfactory early in childhood, can be changed, and it is the pathway, not the baseline, that shows to be of major importance for children to achieve a fit state. In conclusion, children show different rates of change in fitness development over childhood; and having a positive developmental trajectory of fitness predicts a fitter state at the end of childhood, independent of the initial values at 6-years-of-age.