BACKGROUND: Homeostasis model assessment of insulin resistance (HOMAIR) had been proposed as a simple and inexpensive alternative to other complex procedures measuring insulin resistance. We evaluated the validity of HOMAIR, comparing to total glucose disposal rate measured by euglycemic clamp test in 63 subjects with normal glucose tolerance, 21 with impaired glucose tolerance and 47 with type 2 DM. METHODS: HOMAIR and HOMA cell function (Homeostasis model assessment of cell function) were calculated with formula described by Matthews [HOMAIR: fasting insulin ( U/mL) X fasting glucose (mmol/L) / 22.5, HOMA cell function: 20 X fasting insulin ( U/mL) / (fasting glucose (mmol/L) - 3.5)]. 2-hour euglycemic (5 mmol/L) hyper insulinemic (717 pmol/L) clamp test were carried out. RESULTS: The strong inverse correlation (r=-0.658, <0.001) was shown between log transformed HOMAIR and total glucose disposal rates. The agreement of two methodes in the categorization according to insulin resistance was moderate (weighed kappa=0.45). The magnitude of correlation coefficients were smaller in subjects with lower BMI (BMI < 23.7 kg/m2, r = -0.441 vs BMI > or = 23.7 kg/m2, r = -0.693, p = 0.0183), lower HOMA cell function (HOMA cell function < 57.2, r = -0.514 vs HOMA cell function > or = 57.2, r = -0.773, p = 0.0091) and higher fasting glucose levels (fasting glucose < 102 mg/dL, r = -0.697 vs fasting glucose > or = 102 mg/dL, r = -0.59, p = 0.0735). The results of correlation analysis was not significant in diabetics with lower BMI. CONCLUSION: Limitation of validity of HOMAIR should be carefully considered in subjects with lower BMI and lower fasting insulin to glucose levels, such as lean type 2 diabetes with insulin secretory defects.