Recently, the diagnostic criteria for type 2 diabetes mellitus have been changed, but there are disagreements about which measurements should be used. In contrast to the American Diabetes Association (ADA), The World Health Organization (WHO) still recognizes fasting and 2-h glucose concentrations measured on either plasma or whole blood as diagnostic tools. Insulin sensitivity and insulin secretion are both assumed to be involved in the pathogenesis of type 2 diabetes. The oral glucose tolerance test (OGTT) for estimating insulin sensitivity and secretion is increasingly used, e.g. in intervention trials. The objectives of this study were to estimate the coefficients of intra-individual variation (CVw) of blood glucose and serum insulin concentrations from an OGTT as well as indices of insulin sensitivity (HOMA) and insulin secretion (delta insulin30/delta glucose30) derived from this test. Following duplicate OGTTs with a median interval of 13 days (range 1-87 days), the analytical, inter-individual, and intra-individual coefficients of variation were calculated by nested ANOVA. The CVw for fasting blood glucose (7%) was considerably lower than that for 2-h post-load glucose (15%), which was again lower than for the insulin concentrations and indices of insulin sensitivity and secretion. In conclusion, the intra-individual variation is larger for 2-h post-load glucose than for fasting glucose and may question the continued use of the 2-h post-load glucose value in the diagnosis of type 2 diabetes.