We have recently described a novel phenotype in a group of subjects with type 1 diabetes that is manifested by glucose >11.1 mmol/l 120 min after an oral glucose load, but with normal fasting glucose levels. We now describe the metabolic characteristics of these subjects by comparing parameters of islet hormone secretion and glucose disposal in these subjects to age-matched nondiabetic control subjects. The patients with type 1 diabetes had fasting glucose, insulin, and glucagon values similar to those of control subjects. Additionally, the insulin secretory response to intravenous arginine at euglycemia was similar in the control and diabetic groups (264 ± 33.5 and 193 ± 61.3 pmol/l; P = 0.3). However, marked differences in β-cell function were found in response to hyperglycemia. Specifically, the first-phase insulin response was lower in diabetic subjects (329.1 ± 39.6 vs. 91.3 ± 34.1 pmol/l; P < 0.001), as was the slope of glucose potentiation of the insulin response to arginine (102 ± 18.7 vs. 30.2 ± 6.1 pmol/l per mmol/l; P = 0.005) and the maximum insulin response to arginine (2,524 ± 413 vs. 629 ± 159 pmol/l; P = 0.001). Although plasma levels of glucagon-like peptide (GLP)-1 and gastric inhibitory peptide (GIP) did not differ between control and diabetic subjects, the incretin effect was lower in the diabetic patients (70.3 ± 5.4 vs. 52.1 ± 5.9%; P = 0.03). Finally, there was a lack of suppression of glucagon in the patients after both oral and intravenous glucose administration, which may have contributed to their postprandial hyperglycemia. Glucose effectiveness did not differ between patients and control subjects, nor did insulin sensitivity, although there was a tendency for the patients to be insulin resistant (9.18 ± 1.59 vs. 5.22 ± 1.17 pmol · l⁻¹ · min⁻¹; P = 0.08). These data characterize a novel group of subjects with type 1 diabetes manifested solely by hyperglycemia following an oral glucose load in whom islet function is normal at euglycemia, but who have marked defects in both α- and β-cell secretion at hyperglycemia. This pattern of abnormalities may be characteristic of islet dysfunction early in the development of type 1 diabetes.