The prevalence of obesity in British infants in the early 1970s has been attributed to overfeeding and early weaning but cannot be totally explained by this. Obesity in early life is a consequence of the cyclical nature of fat deposition in childhood. Fat deposition in early infancy is succeeded by a lean tissue deposition phase and then a further acceleration of fat accretion--the adiposity rebound--in early school years. The age at onset of adiposity rebound relates more closely to later obesity than does the nutritional status in infancy. The propensity to deposit fat readily is probably genetically determined although dependent for expression on the nurturing environment. Since the number of young children at risk of obesity is so great, measures that modify national diet and activity rather than concentrate on controlling individual fat deposition, may be the most efficient way of preventing obesity in adult life.