OBJECTIVE: Critical stages in childhood are suspected for adult obesity. We sought to identify (i) whether risk of adult obesity is influenced by childhood socioeconomic conditions in addition to those in adulthood; and (ii) whether conditions in childhood act independently or through their association with education or parental obesity.DESIGN: Longitudinal, 1958 British birth cohort.SUBJECTS: A total of 11 405 men and women followed to age 33 y.MEASUREMENTS: Social class at birth and ages 7, 11, 16, 23 and 33 y. Obesity (BMI⩾30) at age 33 y.RESULTS: Social class was related to obesity, cross-sectionally at ages 16 (women), 23 and 33 y, but not at younger ages. In analysis of adult obesity (age 33 y) and social class at five life stages, class at age 7 y significantly predicted obesity for women (adjusted odds ratio (OR)=1.31, that is, the odds increased by 31% for each decrease in social class). For men, class at birth and age 23 y predicted adult obesity (adjusted OR=1.19 and 1.16, respectively). Education was also associated with adult obesity, increasing the odds by 30% (men) and 35% (women) for each decrease in qualification level. Adjustment for education level and parental BMI did not abolish the effect on adult obesity of class at age 7 y among women, nor of class at birth among men, while class at age 23 y reduced to borderline significance.CONCLUSIONS: Cross-sectional associations for social class and obesity can be misleading and obscure effects of childhood socioeconomic conditions. Influences around birth to age 7 y have a long-lasting impact on the risk of adult obesity.