The role of early undernutrition as a risk factor for adult obesity is supported by data from epidemiologic, clinical, and animal studies. In experimental animals, maternal energy restriction results in pups of reduced body size but with increased fat depots and hyperphagia. Similarly, reduced lean mass with relative increase in fat mass in the newborn have been described in gestational energy restriction in human populations. When coupled with the appropriate environmental conditions (widely available energy-dense foods, low energy expenditure) those factors predispose a person to excess weight gain and body fat accumulation in adult life. This combination of early biologic factors with environmental factors is best exemplified in the situation of populations from developing countries undergoing economic and nutritional transition, where high prevalence of low birth weight now is combined with an urban, sedentary lifestyle and increased access to low-cost, energy-dense foods. Thus reducing the prevalence of intrauterine growth retardation and early undernutrition should be an important component of our effort to control the worldwide obesity epidemic.