Insulin resistance was assessed by euglycaemic clamp studies in matched groups of MODY, classical NIDDM patients and non-diabetic control subjects. The MODY patients metabolised less glucose (4.8 +/- 0.3 mg/Kg/min) than the classical NIDDM patients (7.0 +/- 0.5 mg/Kg/min) at an insulin infusion rate of 1.0 mU/Kg/min (p less than 0.05). At an insulin infusion rate 10 mU/Kg/min the differences between the MODY and the classical NIDDM patients were not significant. At both infusion rates the two diabetic groups metabolised less glucose than the control subjects. The results indicate that despite their younger age, the patients with MODY are more insulin resistant than the patients with classical NIDDM.