Aim: This study explored weight trends among children aged 4, 7, 11, 14 and 17years in Jonkoping County Sweden, from 2004 to 2015. Methods: The study had a repeated cross-sectional design, and body mass index (BMI) was calculated based on height and weight measurements collected from child health and school health records. The prevalence of thinness, overweight and obesity was estimated with international cut-offs, with linear trends calculated separately for boys and girls. Results: There were 190 965 measurements of BMI and these covered 82-97% of the younger children and 55-69% of the older children during the study period. The prevalence of thinness varied between 0.2% and 2.2% across time and age groups and did not change over the study period. There was a small decrease in overweight among both girls and boys aged four years. There were increasing trends in overweight and obesity in both girls and boys aged 11 and 14years of age and a sharp increase among 17-year-old boys, with 7.3% obese in 2014/2015 and 3.6% in 2004/2005. Conclusion: The prevalence of obesity decreased from 2004 to 2015 or was stable in younger Swedish children, but increased among older children, with a large increase in adolescent boys.