The homeostasis model assessment-insulin resistance(HOMA-IR) index has been proposed to assess insulin resistance, which plays an important role in the pathogenesis of type 2 diabetes mellitus. However, it has not generally introduced into the outpatient care in Japan, because of lack in definite reference values. In this paper we studied the precision in the assay of immuno-reactive insulin and then the HOMA-IR index was determined in 90 healthy individuals and 281 type 2 diabetic patients. The reference value was calculated to be 1.77 +/- 0.44(mean +/- SD) in healthy individuals without obesity. Receiver operating characteristic curve analysis gave the cut-off values discriminating between healthy and diabetic subjects to be 2.53, 3.50 and 3.00 in non-obese, obese and total individuals, respectively. These values should be available for the diagnosis of insulin resistance in type 2 diabetes mellitus.