For more than a decade, the fetal programming hypothesis has taught that insulin resistance and its associated metabolic disturbances result from poor gestational environment, for which low birth weight is a surrogate. Low birth weight, however, is now uncommon in industrialized societies. We have investigated the relevance of birth weight, "catch-up" weight, and current weight to insulin resistance in 300 contemporary British children. Insulin resistance at 5 years was not related to birth weight but was correlated with current weight and weight catch-up in both sexes, more strongly so in girls (r = 0.33, P < 0.001 vs. r = 0.18, P = 0.03), who were intrinsically more insulin-resistant than boys. Weight change merely co-correlated with current weight (r = 0.67, P < 0.01 in both sexes) and did not improve on the prediction of insulin resistance. Most important, insulin resistance at 5 years was the same in children of heavier birth weight, whose weight SD score had not changed, as in those of lighter birth weight, matched for current weight, who had experienced so-called catch-up (boys 0.89 and 0.88 units, respectively, P = 0.96; girls 1.26 and 1.13 units, P = 0.41). Insulin resistance in contemporary children seems to be a function of excess current weight rather than of low birth weight or change in weight.