Objective To determine whether the same relationships between early-life risk factors and socioeconomic status (SES) with childhood body mass index (BMI) are observed in a modern cohort (2000) compared with a historic cohort (1947). Study design The relationships between early-life factors and SES with childhood BMI were examined in 2 prospective birth cohorts from the same region, born 50 years apart: 711 children in the 1947 Newcastle Thousand Families Study (NTFS) and 475 from the 2000 Gateshead Millennium Study (GMS). The associations between birth weight, breastfeeding, rapid infancy growth (0-12 months), early-life adversity (0-12 months), and parental SES (birth and childhood) with childhood BMI z-scores and whether overweight/obese (BMI >91st percentile using UK 1990 reference) aged 9 years were examined using linear regression, path analyses, and logistic regression. Results In the NTFS, the most advantaged children were taller than the least (+0.91 height z-score, P = .001), whereas in GMS they had lower odds of overweight/obese than the least (0.35 [95% CI 0.14-0.86]). Rapid infancy growth was associated with increased BMI z-scores in both cohorts, and with increased likelihood of overweight/obese in GMS. Conclusions This study suggests that children exposed to socioeconomic disadvantage or who have rapid infancy growth in modern environments are now at lower risk of growth restriction but greater risk of overweight.