Adults who had low birthweight and were thin at birth have an increased risk of Type 2 diabetes and impaired glucose tolerance. To discover whether thinness at birth is associated with reduced glucose tolerance in children, 250 7-year-old children underwent an abbreviated oral glucose tolerance test. Children who were thin at birth, as measured by a low ponderal index (birthweight length-3) had higher plasma glucose concentrations. Plasma glucose concentration 30 min after a glucose load rose by 0.07 mmol l-1 (95% confidence interval 0.00 to 0.14; p = 0.04) for every unit (kg m-3) fall in ponderal index. Children in the lowest quarter of the distribution of ponderal index (23 kg m-3 or less) had a mean 30 min plasma glucose concentration of 8.49 mmol l-1 compared to a mean of 7.97 mmol l-1 for those in the highest quarter (> 27.5 kg m-3). These associations were independent of duration of gestation, gender, social class or the child's current weight. This is consistent with the hypothesis that Type 2 diabetes originates in utero.