Williams syndrome (WS) is a rare, multisystemic genomic disorder showing a high prevalence of impaired glucose metabolism in adulthood. The reason for this association is unknown, though hemizygosity for genes mapping to the WS chromosome region has been implicated. Twenty-two Italian young adults with WS (13 females, 9 males) were studied. A 75 g oral glucose tolerance test (OGTT) was performed and β-cell function was estimated with Homeostasis Model Assessment (HOMA)-B%, Insulinogenic Index, and corrected insulin response whereas insulin sensitivity was assessed with HOMA-Insulin Resistance Index, Quantitative Insulin Check Index, and composite Insulin Sensitivity Index. One patient had known diabetes mellitus (DM), whereas impaired glucose tolerance (IGT) was diagnosed in 12 patients and DM in one (63.6% prevalence of impaired glucose metabolism). IGT patients were more insulin resistant than those with normal glucose tolerance (NGT), whereas β-cell function was unchanged or increased. Islet autoimmunity was absent. Logistic regression showed that impaired glucose metabolism was not associated with age, body mass index (BMI), or family history of DM. β-cell function, insulin sensitivity, and post-load insulin levels did not differ between WS patients with NGT and healthy controls comparable for gender, age, and BMI, though WS–NGT patients had higher post-load glucose values. These data confirm the high prevalence of impaired glucose metabolism in WS young adults, thus suggesting the need for screening these patients with OGTT. IGT is associated with reduced insulin sensitivity, but not with impaired β-cell function, islet autoimmunity, and traditional risk factors for type 2 DM. © 2013 Wiley Periodicals, Inc.