An increased prevalence of impaired glucose homeostasis (IGH) and diabetes mellitus is reported in monosomy X, or Turner syndrome (TS). To determine whether IGH is an intrinsic feature of this syndrome, independent of obesity or hypogonadism, we compared results of a standard oral glucose challenge in age- and body mass index-matched women with TS and with karyotypically normal premature ovarian failure (POF). Fasting glucose levels were normal in both groups, but glucose values after oral glucose challenge were higher in TS [2-h glucose, 135 ± 36 mg/dl (7.5 ± 2.0 mmol/liter) in TS and 97 ± 18 mg/dl (5.4 ± 1.0 mmol/liter) in POF; P < 0.0001]. Glucose-stimulated insulin secretion was lower in TS; e.g. the initial insulin response (ΔI/ΔG30) was decreased by 60% compared with POF (P < 0.0001). We also compared responses to a standard iv glucose tolerance test in women with TS and in age- and body mass index-matched normal women and found that the insulin area under the curve was 50% lower in women with TS (P = 0.003). Insulin sensitivity measured by the quantitative insulin sensitivity check index was higher in women with TS compared with both control groups. Thus, IGH is not secondary to obesity or hypogonadism in TS, but it is a distinct entity characterized by decreased insulin secretion, suggesting that haploinsufficiency for X-chromosome gene(s) impairs β-cell function and predisposes to diabetes mellitus in TS.