BACKGROUND: Obesity is an increasingly prevalent nutritional disorder throughout the world and is a risk factor for many chronic diseases. The prevalence of obesity increases with age. OBJECTIVE: The objective was to evaluate the associations between BMI at 31 y of age and family social class during early childhood, maternal body mass index (BMI) before pregnancy, BMI at birth and at 1 and 14 y of age, and age at menarche. DESIGN: This was a longitudinal study of the northern Finland birth cohort for 1966. Subjects were measured at birth and at 1, 14, and 31 y of age. The analysis was restricted to individuals for whom BMI data were available for all measurement points (n = 2876 males and 3404 females). RESULTS: The mean BMI at birth was highest in offspring from the highest social classes, but BMI was inversely related to social class at 1 y. BMI, the waist-to-hip ratio, and the proportion of obese subjects were inversely related to social class at 31 y. The heavier the mother, the heavier the offspring from birth to 31 y. The paired analyses between maternal BMI and daughter's BMI at 31 y showed no significant difference in BMI after adjustment for the age difference. BMI at 14 y was the most important predictor of BMI at 31 y. Early menarche in females was associated with a higher BMI at 14 and 31 y. CONCLUSIONS: Differences in BMI by social class are formed at least partly during early childhood. Low social class of the child's family, a high maternal BMI before pregnancy, a high BMI during adolescence, and early menarche are predictors of obesity in adulthood.