OBJECTIVES: To examine how population-level socioeconomic health inequalities developed during childhood, for children born at the turn of the 21st century and who grew up with major initiatives to tackle health inequalities (under the New Labour Government). SETTING: The UK. PARTICIPANTS: Singleton children in the Millennium Cohort Study at ages three (n=15 381), five (n=15,041), seven (n=13,681) and eleven (n=13,112) years. PRIMARY OUTCOMES: Relative (prevalence ratios (PR)) and absolute health inequalities (prevalence differences (PD)) were estimated in longitudinal models by socioeconomic circumstances (SEC; using highest maternal academic attainment, ranging from ‘no academic qualifications’ to ‘degree’ (baseline)). Three health outcomes were examined: overweight (including obesity), limiting long-standing illness (LLSI), and socio-emotional difficulties (SED). RESULTS: Relative and absolute inequalities in overweight, across the social gradient, emerged by age five and increased with age. By age eleven, children with mothers who had no academic qualifications were considerably more likely to be overweight as compared with those with degree-educated mothers (PR=1.6 (95 per cent CI 1.4 to 1.8), PD=12.9 per cent (9.1 per cent to 16.8 per cent)). For LLSI, inequalities emerged by age seven and remained at eleven, but only for children whose mothers had no academic qualifications (PR=1.7 (1.3 to 2.3), PD=4.8 per cent (two per cent to 7.5 per cent)). Inequalities in SED (observed across the social gradient and at all ages) declined between three and eleven, although remained large at eleven (eg, PR=2.4 (1.9 to 2.9), PD=13.4 per cent (10.2 per cent to 16.7 per cent) comparing children whose mothers had no academic qualifications with those of degree-educated mothers). CONCLUSIONS: Although health inequalities have been well documented in cross-sectional and trend data in the UK, it is less clear how they develop during childhood. We found that relative and absolute health inequalities persisted, and in some cases widened, for a cohort of children born at the turn of the century. Further research examining and comparing the pathways through which SECs influence health may further our understanding of how inequalities could be prevented in future generations of children. [Abstract]