To clarify whether quantitative insulin sensitivity check index (QUICKI) is useful as an indicator of insulin resistance in Japanese metabolically obese, normal-weight (MONW, body mass index (BMI) <25 and visceral fat area (VFA) > or = 100 cm2 by abdominal computed tomography scanning) subjects with normal glucose tolerance (NGT). Insulin resistance was measured by QUICKI and euglycemic hyperinsulinemic clamp study (clamp IR) in 27 MONW and 27 normal subjects (BMI <25 and visceral fat area <100 cm2). QUICKI (P<0.01) and clamp IR (p<0.01) were significantly decreased in MONW subjects compared with normal subjects. QUICKI was significantly associated with VFA [MONW subjects: r = -0.459, p<0.02; all (MONW and normal) subjects: r = -0.506, p<0.0001] and with the serum levels of triglycerides (MONW subjects: r = -0.386, p<0.05; all subjects: r = -0.505, p<0.001) in MONW and all subjects. There were significant correlations between clamp IR and VFA (MONW subjects: r = -0.562, p<0.01; all subjects: r = -0.575, p<0.0001), fasting serum levels of insulin (MONW subjects: r = -0.673, p<0.001; all subjects: r = -0.619, p<0.0001) or serum levels of triglycerides (MONW subjects: r = -0.485, p<0.02; all subjects: r = -0.565, p<0.0001) in MONW and all subjects. QUICKI was significantly correlated with clamp IR in MONW (r = 0.754, p<0.0001) and in all subjects (r = 0.568, p<0.0001). QUICKI may be an useful method for assessing insulin resistance in Japanese MONW subjects with NGT.