To metabolically characterize patients with slowly progressing autoimmune diabetes (LADA) of short duration we measured insulin, C peptide, and glucagon responses to glucose and arginine at three blood glucose levels (fasting and 14 and 28 mmol/L) in 11 patients with LADA, 11 patients with type 2 diabetes, and 14 healthy control subjects matched for age and body mass index. The acute insulin response to arginine was impaired in LADA vs. type 2 diabetes at all glucose levels, with the greatest impairment in the maximally stimulated insulin concentrations (P < 0.04). In contrast, β-cell sensitivity to glucose was unaltered in LADA and type 2 diabetes. The glucagon concentrations were elevated in both LADA and type 2 diabetic patients compared with healthy control subjects (P < 0.02), but did not differ between the diabetic groups. In conclusion, patients with LADA share insulin resistance with type 2 diabetic patients, but display a more severe defect in maximally stimulated β-cell capacity than patients with type 2 diabetes.