In adults low birthweight and thinness at birth are associated with increased risk of glucose intolerance and non-insulin-dependent diabetes mellitus. We have examined the relations between size at birth (birthweight, thinness at birth) and levels of plasma glucose and serum insulin in children, and compared them with the effects of childhood size. We performed a school-based survey of 10-11-year-old British children (response rate 64%) with measurements made after an overnight fast. One group of children (n = 591) was studied fasting while the other (n = 547) was studied 30 min after a standard oral glucose load (1.75 g/kg). Serum insulin was measured by a highly specific ELISA method. Birthweight was assessed by maternal recall and thinness at birth using birth records. Neither fasting nor post-load glucose levels showed any consistent relationship with birthweight or ponderal index at birth. After adjustment for childhood height and ponderal index, both fasting and post-load insulin levels fell with increasing birthweight. For each kg increase in birthweight, fasting insulin fell by 16.9% (95% confidence limits 7.1-25.8%, p = 0.001) and post-load insulin by 11.6% (95% confidence limits 3.5-19.1%, p = 0.007). However, the proportional change in insulin level for a 1 SD increase in childhood ponderal index was much greater than that for birthweight (27.2% and -8.8%, respectively, for fasting insulin). We conclude that low birthweight is not related to glucose intolerance at 10-11 years, but may be related to the early development of insulin resistance. However, in contemporary children obesity is a stronger determinant of insulin level and insulin resistance than size at birth.