In a representative sample of 895 schoolchildren, aged between 9 years 10 months and 11 years 2 months, the risk of being overweight or obese was compared between those who had gained weight rapidly during infancy and those whose weight gain had been normal. A substantially increased risk ratio was found only in boys for whom a correlation analysis showed that the total weight gain during the first year of life was associated with the total body mass in relation to height, more or less independently of the degree of fatness at 10 1/2 years of age. In girls, a direct but very weak association was found between weight gain in infancy and the degree of fatness at 10 1/2 years. The implications of these findings with respect to aetiology and the possibilities of prevention are briefly discussed.