This study was designed to evaluate the influence of age, duration of diabetes, relative body weight and glycaemic control on beta-cell function and insulin sensitivity in 250 patients with onset of nonketotic diabetes between the age of 35 and 70 years (Type 2 diabetes). Beta-cell function was assessed by measuring serum C-peptide concentrations after 1 mg of glucagon iv. It was not influenced by age, age at onset of diabetes nor by the duration of the disease. This suggests that progressive deterioration of beta-cell function with time is not a consistent finding in Type 2 diabetes. Insulin sensitivity, measured as the glucose disappearance rate, KITT, in response to iv-insulin, was not significantly influenced by age or age at onset, but decreased consistently with the duration of the disease (P less than 0.001). Beta-cell function was not correlated to fasting blood glucose and HbA1 concentrations. In contrast, there was a strong inverse relationship between glycaemic control and insulin sensitivity (P less than 0.001) indicating that decreased insulin sensitivity contributes to poor glycaemic control in these patients. Attempts to improve glycaemic control in patients with Type 2 diabetes should therefore include means to improve insulin sensitivity.