The aetiology of obesity in children is studied here with the aid of a material consisting of 40 monozygotic and 61 dizygotic twins. The importance of heredity, early nutrition, and environment during preschool age is analysed. Genetic factors, studied by comparison of intra-pair differences in monozygotic and dizygotic twins, apparently play a decisive role in the origin of obesity. Analysis of the relation between birth weight and later development of subcutaneous fat in different types of twins does not show that intra-uterine nutrition is of aetiological importance. The comparison at age seven of groups of ordinary siblings and twins of different types does not support the idea that the pre-school environment is of importance in aetiology. Such influence seems to act mainly as a modifying factor.