Explanations for the trend of increasing childhood obesity have yet to be identified. The authors examined members of the 1958 British birth cohort (age 7 years, n = 8,552) and offspring (ages 4-9 years, n = 1,889) born to mothers under age 30 years to establish whether risk factors for childhood obesity have changed over time (1965-1991). The authors applied multilevel linear and logistic models that account for within-family correlations in order to examine associations between risk factors and childhood body mass index (BMI; weight (kg)/height (m)(2)) (age- and sex-specific standard deviation scores and overweight/obesity) in the 2 populations. The authors found that the prevalence of overweight/obesity had increased by more than 50% between generations. Parental BMI and full-time maternal employment were both positively associated with offspring BMI in childhood (e.g., an increase of 0.4-0.5 units for maternal employment); these associations had strengthened between generations. There was evidence of a widening social gap in childhood obesity: Indicators of lower socioeconomic position showed either no association or a protective effect in cohort members but were associated with increased BMI in offspring. Prevalences of parental obesity and maternal employment had increased. Socioeconomic factors had improved across generations. Parental obesity, maternal employment, and socioeconomic factors may play an increasing role in the childhood obesity epidemic.