Homeostasis model assessment of insulin resistance (HOMA-IR) is a simple and useful method for evaluating insulin sensitivity. But it is difficult to apply to type2 diabetes patients treated with insulin. We have devised a method for measuring HOMA-IR and investigated the validity of HOMA-IR for evaluating insulin sensitivity in patients with type 2 diabetes on insulin therapy. In the first arm of the study, 19 poorly controlled diabetic subjects were treated with insulin and underwent euglycemic clamp study. Then the relationship between insulin resistance index assessed by the clamp test (clamp-IR) and HOMA-IR was investigated in these subjects. Log transformed HOMA-IR correlated with log transformed M/I values derived from the standard euglycemic clamp (r=-0.753, p=0.002). In the second arm of the study, we investigated the relationship between HOMA-IR and various clinical parameters in 156 patients with poorly controlled diabetes after glycemic control. Log transformed HOMA-IR correlated negatively with age (r=-0.292, p=0.0002), HDL-C (r=-0.342, p<0.0001), log transformed serum adiponectin (r=-0.309, p=0.0006) and log transformed KITT (r=-0.264, p=0.0009), and positively with body mass index (r=0.499, p<0.0001), waist circumstance (r=0.461, p<0.0001), visceral fat area (r=0.401, p<0.0001), diastolic blood pressure (r=0.223, p=0.0054), log transformed triglyceride (r=0.497, p<0.0001), urinary CPR (r=0.216, p=0.0099), ΔCPR of glucagon stimulation test (r=0.496, p<0.0001) and log transformed insulinogenic index (r=0.325, p=0.0002). These results suggest that HOMA-IR is a useful test for the evaluation of insulin sensitivity even in patients with type 2 diabetes treated with insulin.