The aim of the present study was to investigate whether or not residual B-cell function could be related to insulin sensitivity as well as to duration of disease, insulin requirement, and indices of metabolic control in a population of Type 1 (insulin-dependent) diabetic patients. A positive correlation was found between fasting C-peptide and age at onset of diabetes, whereas a negative relationship occurred between C-peptide and duration of disease. Fasting C-peptide negatively correlated also to mean daily plasma glucose, 24-h glycosuria, and fasting free fatty acid concentration. Moreover, a negative correlation was found between C-peptide and daily insulin requirement. Conversely, a positive relationship occurred between C-peptide levels and the parameter we used for estimating insulin sensitivity, i.e. glucose disappearance rate after i.v. insulin injection. These results once more emphasize the importance of residual B-cell function in Type 1 (insulin-dependent) diabetes mellitus, and suggest that the residual endogenous insulin secretion might play an important role in glucose homeostasis of Type 1 diabetes by influencing the sensitivity to insulin.