We previously reported that the adiponectin-leptin (A/L) ratio was more efficacious as a parameter of insulin resistance than adiponectin or leptin alone, and a more sensitive and reliable marker of insulin resistance than homeostasis model assessment (HOMA-R) as the fasting plasma glucose (FPG) level elevated in type 2 diabetes mellitus. In this study, we examined the usefulness of the A/L ratio as compared to HOMA-R for assessing insulin resistance in Japanese subjects without hyperglycemia. A total of 411 Japanese adults without hyperglycemia (205 men, aged 49 +/- 10 years; 206 women, aged 48 +/- 10 years) were enrolled. We investigated the correlation between fasting serum insulin level, FPG, leptin or adiponectin, and body mass index (BMI), fat mass (FM), triglycerides (TGs), high-density lipoprotein (HDL) cholesterol, or preheparin serum lipoprotein lipase (LPL) as parameters of insulin resistance. Next, we examined the relationships between parameters of insulin resistance and the A/L ratio or HOMA-R. By simple regression of the correlation between serum insulin level, FPG, leptin or adiponectin, and each parameter of insulin resistance, the best correlation coefficients were seen in leptin (men, r = 0.501; women, r = 0.667) as compared with BMI, in leptin (men, r = 0.658; women, r = 0.747) as compared with FM, in adiponectin (r = -0.285) in men and leptin (r = 0.299) in women as compared with TGs, in adiponectin (men, r = 0.405; women; r = 0.442) as compared with HDL cholesterol, and in adiponectin (men, r = 0.228; women, r = 0.452) as compared with LPL. By simple regression of the correlation between A/L ratio or HOMA-R and each parameter of insulin resistance, the highest correlation coefficients were seen with the A/L ratio except HDL cholesterol in men. Next, we carried out multiple linear regression to analyze the association between A/L ratio or HOMA-R and FM, TGs, HDL cholesterol, and LPL, excluding BMI, simultaneously. In men, the A/L ratio was significantly correlated with FM and TGs, and HOMA-R was significantly correlated with FM. This model explained 34% of the variance in the A/L ratio and 17% of the variance in HOMA-R. In women, the A/L ratio was significantly correlated with FM and LPL, and HOMA-R was significantly correlated with FM and LPL. This model explained 39% of the variance in A/L ratio and 14% of the variance in HOMA-R. In conclusion, the present study suggested that the A/L ratio might be more useful than HOMA-R to accurately assess insulin resistance in subjects without hyperglycemia.