We wanted to investigate whether time to steady state was reached within 2 h of insulin infusion during isoglycemic hyperinsulinemic clamp, comparing the glucose uptake index (M/IG) with Bergman's insulin sensitivity index (Sip). We performed a 2-h oral glucose tolerance test and a 3-h isoglycemic hyperinsulinemic clamp in 26 young, healthy subjects and 43 elderly patients with unmedicated essential hypertension and left ventricular hypertrophy. The 3-h Sip correlated strongly with the 2-h M/IG in the patients (r = 0.88, p < 0.001) as well as in the healthy subjects (r = 0.96, p < 0.001) with relatively narrow limits of agreement in the patients. However, during the third hour of insulin infusion, M/IG (10.0 vs 12.2 12 x kg-1 x min-1 x mmol-1, p < 0.001) as well as Sip (7.1 vs 9.412 x kg-1 x min-1 x mmol-1, p < 0.001) increased significantly in the patients, but not in the healthy subjects. Because the 2-h M/IG correlated strongly with the 3-h Sip with relatively narrow limits of agreement, it is a good measure of insulin sensitivity. However, a 2-h clamp results in lower insulin sensitivity values in elderly, hypertensive patients due to the fact that steady state is not reached, demonstrating a higher prevalence of insulin resistance in such a population.