Background The aim of the present study was to investigate whether or not insulin resistance is present in hypertrophic cardiomyopathy (HCM), and also whether it is related to the clinical manifestations of HCM. Methods and Results The study group comprised 55 patients with HCM, 35 with essential hypertension (EHT) and 15 normotensive control subjects (NC). An insulin resistance index was estimated using the homeostasis model assessment (HOMA-IR) of fasting insulin - glucose interactions. In patients with HCM, prognosis and cardiovascular events were also checked. The HOMA-IR values in the HCM group (2.90±1.22) were significantly higher than those in the EHT (1.69±0.77) and NC groups (0.91±0.24). The HOMA-IR values in the EHT group were significantly higher than those in the NC group. Multiple regression analyses determined that left ventricular pressure gradient without provocation (p<0.0001), interventricular septal thickness (p=0.0143) and body mass index (p=0.0412) were independent determinants of insulin resistance in patients with HCM. During a mean follow-up of 105±50 months, 4 patients with HCM died suddenly and all of them had high HOMA-IR values. Conclusions The results suggest that patients with HCM without apparent diabetes mellitus or hypertension have insulin resistance and that insulin resistance may be related to the manifestations of HCM. (Circ J 2004; 68: 650 - 655)