Background: The results of epidemiologic studies have linked birth size to adult glucose intolerance. Objective: We investigated this association in a genetically homogeneous population with higher birth weights and a lower prevalence of type 2 diabetes than previously studied. Design: The subjects were 2362 men and 2286 women aged 33-65 y. Size at birth was obtained from the National Archives of Iceland. Data for adult anthropometry, fasting blood glucose, and blood glucose after an oral glucose load came from the randomized prospective Reykjavík Study. Results: Postchallenge glucose concentrations were inversely related to birth weight and length in men and inversely related to birth weight and ponderal index in women (P < 0.001). This association was mainly found among those within the highest one-third of adult body mass index values. In men, the prevalence of dysglycemia was lower with increasing weight (P = 0.04) and length (P = 0.003) at birth but there was no relation of dysglycemia to ponderal index. For women, there was no linear trend for dysglycemia in relation to size at birth but the relation with birth length was U shaped. Conclusions: Greater birth weight and length appear to offer a protective effect against glucose intolerance. Adult overweight or obesity enhances the risk associated with low birth weight and length. Because the population studied has higher birth weights and a lower prevalence of type 2 diabetes than are found in neighboring countries, it is possible that decreasing the number of low-birth weight infants might help to stem the increasing prevalence of type 2 diabetes worldwide.