Objective: Racial/ethnic patterning in the risk of obesity and overweight has been observed in early childhood; however, little research has compared these disparities between the UK and US using detailed ethnic classifications. We use comparable nationally representative cohort studies to examine racial/ethnic disparities in mean BMI and in the odds of obesity/overweight in the UK and US. The contribution of socio-demographic, cultural, and family routine factors are assessed. Methods: Data on BMI, obesity and overweight in 5-year-old children from the MCS and ECLS-B were examined. We investigated race/ethnic disparities in mean BMI and in the odds of obesity and overweight, as compared to normal weight. We assessed the independent contribution of socio-demographic, cultural, and family routine factors to observed disparities. Results: In the UK, after adjustment for socio-demographic, cultural and family routine factors, and maternal BMI, we found Black Caribbean children to have higher odds (OR=1.7, CI=1.1-2.6) and Pakistani children to have lower odds of obesity (OR=0.60, CI=0.37-0.96), and Black African children were more likely to be overweight (OR=1.40, CI=1.04-1.88). In the US, in fully-adjusted models, there were no race/ethnic disparities in children’s odds of obesity and overweight. Conclusions: Disparities for Bangladeshi children in the UK and Mexican, other Hispanic, and American Indian children in the US can be explained by socioeconomic disadvantage, whereas a range of cultural and family characteristics partially explain disparities for other groups in the UK. Future public health initiatives focused on reducing risk of overweight and obesity should consider the diverse socio-economic and cultural profiles of all race/ethnic groups.