Purpose The study’s purposes were to 1) assess changes in cardiorespiratory fitness (CRF) from 1998 to 2010, controlling for decimal age and body mass index (BMI), and 2) repeat the analysis in cohorts from 2005 to 2010, controlling for maturation, deprivation, and BMI. Methods : A total of 27,942 (n= 14,247 boys) 9- to 10.9-yr-old participants from one UK city were included in this serial cross-sectional study from 1998–1999 to 2009–2010. An indices of multiple deprivation (IMD) score was assigned to each participant on the basis of home postcode. Stature, sitting stature, and body mass were estimated. BMI and somatic maturity were calculated. Performance on the 20-m multistage shuttle run test (20mSRT) was used to estimate CRF (total shuttles). One-way ANCOVAs were completed to assess temporal trends in CRF, separately by sex. Model 1 assessed changes from 1998 to 2010 and included decimal age and BMI as covariates. Model 2 assessed changes from 2005 to 2010 and included maturity, IMD, and BMI as covariates. Results : Results indicate that 20mSRT performance has declined in UK schoolchildren. An annual decline of 1.34% and 2.29% was observed in boys and girls, respectively. In model 1, for boys, the baseline cohort performed better than all other groups with the exception of the 1999–2000 group. For girls, declines in 20mSRT performance were observed from 2003 onward. In model 2, for boys, the 2007–2008 and 2008–2009 cohorts completed fewer 20mSRT shuttles than all other groups. For girls, the 2007–2008 and 2008–2009 cohorts also performed worse than all other years. Conclusions : The decline in CRF suggests children in the more recent SportsLinx cohorts may be at an increased risk of car-diometabolic illness in comparison with earlier cohorts. The promotion of vigorous physical activity is urged to promote CRF in children.