Secular changes in body mass index (BMI) and cardiorespiratory fitness (20 m shuttle-run test performance) were assessed in 10-year-old children from an affluent area of England in 1998 (n = 303; 158 boys and 145 girls) and 2008 (n = 315; 158 boys and 157 girls). Girls’ BMI did not change over the 10 year period. There was a significant increase in boys’ BMI (p = 0.02). Cardiorespiratory fitness declined significantly (p<0.001) in both boys (7%) and girls (9%). This study shows a large and worrying decline in cardiorespiratory fitness in children from an affluent area of England.