Introduction Determinants and correlates of a novel index of adipose tissue insulin resistance (AT-IR) (the product of fasting insulin and free fatty acid concentrations) were investigated in Japanese women without diabetes and obesity. Research design and methods Cross-sectional associations of AT-IR with fat mass and distribution, and IR-related cardiometabolic variables were examined in 210 young and 148 middle-aged women whose average body mass index (BMI) was <23 kg/m(2) and waist was <80 cm. Multivariate linear regression analyses were used to identify most important determinants of AT-IR. Results Young and middle-aged women did not differ in AT-IR (3.5 +/- 2.7 and 3.2 +/- 2.1, respectively). In both young and middle-aged women, AT-IR was positively associated with trunk/leg fat ratio, a sophisticated measure of abdominal fat accumulation, fasting plasma glucose (FPG), fasting triglycerides (FTG), serum alanine aminotransferase and gamma-glutamyl-transpeptidase (all p<0.05). Furthermore, in middle-aged but not in young women, AT-IR showed positive associations with BMI, waist, fat mass index, low-density lipoprotein cholesterol, apolipoprotein B and systolic and diastolic blood pressure (BP) (all p<0.05). AT-IR showed no association with hemoglobin A1c, high-density lipoprotein (HDL) cholesterol and apolipoprotein A1 in two groups of women. On multivariate analysis including waist, FPG, FTG, HDL cholesterol and systolic BP as independent variables, FPG, FTG and HDL cholesterol emerged as independent determinants of AT-IR in young women (cumulative R-2=0.141) and waist in middle-aged women (cumulative R-2=0.056). In a model which included trunk/leg fat ratio instead of waist, trunk/leg fat ratio and systolic BP were determinants of AT-IR in middle-aged women (cumulative R-2=0.093). Results did not alter in young women. Conclusions AT-IR may be a simple and useful surrogate index of adipose tissue insulin resistance even in populations without diabetes and obesity.