The aim of this study was to evaluate the prevalence of the metabolic syndrome and its effect on neonatal outcomes in pregnancies with different degrees of hyperglycemia. One hundred and fifty women with gestational diabetes, 100 with one abnormal value on the oral glucose tolerance test, 100 with a normal oral glucose challenge test and 350 with an abnormal challenge test and normal tolerance test were enrolled. The prevalence of the metabolic syndrome was: 0%, 4.9%, 20% and 18% in the normal challenge test, abnormal challenge and normal tolerance test, one abnormal value and gestational diabetes patients, respectively. Offspring birth weights, prevalence of large-for-gestational age babies and icterus were significantly higher in women with an abnormal challenge test (both with a normal tolerance test or one abnormal value or gestational diabetes). Metabolic syndrome was the best predictor of the presence of large-for-gestational age babies in patients with an abnormal challenge and normal tolerance test (OR = 3.15), one abnormal value (OR = 3.53) and gestational diabetes (OR = 4.15). Metabolic syndrome in mid-pregnancy was an independent predictor of macrosomia in women with any degree of gestational hyperglycemia; the oral glucose challenge test identifies pregnancies with metabolic abnormalities and adverse neonatal outcomes also in the presence of a normal oral glucose tolerance test.