ORTEGA, F. B., I. LABAYEN, J. R. RUIZ, E. KURVINEN, H.-M. LOIT, J. HARRO, T. VEIDEBAUM, and M. SJOSTROM. Improvements in Fitness Reduce the Risk of Becoming Overweight across Puberty. Med. Sci. Sports Exerc., Vol. 43, No. 10, pp. 1891-1897, 2011. Purpose: Information about factors related to overweight development in early stages of life is needed for designing useful strategies to prevent overweight and related diseases. Longitudinal studies can contribute to this goal. The present study aimed to identify factors in childhood that determine the development of overweight/obesity in adolescence. Methods: A prospective study in 598 normal-weight Estonian and Swedish children age 9.5 +/- 0.4 yr from the European Youth Heart Study, who were followed during 6 yr, was conducted. Weight and height were measured at baseline and follow-up, and weight status was ascertained according to the international criteria for body mass index. Cardiorespiratory fitness (expressed as (V) over dotO(2max) (mL.kg(-1).min(-1))) was assessed by a maximal bike test. Parents reported their weight, height, and educational level. Results: Being male (vs female) and Estonian (vs Swedish) was related to higher risk for incident overweight/obesity. Change in fitness was a stronger predictor of incident overweight/obesity than childhood fitness, parental overweight, or parental education. The risk of developing overweight/obesity was reduced 10% every 1 mL.kg(-1).min(-1) of (V) over dotO(2max) increase (odds ratio – 0.90 and 95% confidence interval – 0.84-0.95) after adjustment for a set of confounders including baseline body mass index and without differences by gender. Conclusions: Our results suggest that improvements in fitness from childhood to adolescence are associated with a lower risk of becoming overweight/obese in adolescence. The current findings highlight the importance of promoting fitness through physical exercise from early stages in life, as a promising strategy to fight against overweight and obesity. Gender and country differences observed in this study require social and political attention.