Background: Little is known about the progression of obesity from childhood to adolescence. This study aimed to longitudinally examine the obesity status in a cohort of children across their childhood and adolescence, and to identify the factors associated with persistent obesity. Methods: The study used data from School Physical Fitness Award Scheme (SPFAS), a population-based programme in Hong Kong primary and secondary schools. Students were included if they participated in the SPFAS in both 2014 (Primary 1 and 2) and 2018 (Primary 5 and 6). Their anthropometric and physical fitness parameters were analyzed. Results: A total of 18,863 students were included. The baseline prevalence of obesity was 5.7 %. After 4 years, the prevalence increased to 6.7 %. Among those with obesity at baseline, 35.3 % remained obese after 4 years. The addition of baseline physical fitness level did not improve the prediction for persistent obesity. Conclusions: One-third of obese students in junior primary school remained to be obese into adolescence. Their baseline physical fitness level did not improve the predictive value for future obesity. Further studies should investigate the prognostic factors that may influence the natural course of childhood obesity.