Adiponectin has been reported to regulate systemic insulin sensitivity as a part of a broader control mechanism in energy balance. However, it is not clear whether adiponectin exerts its positive effects on insulin sensitivity equally in a wide range of obesity. We investigated the association of plasma adiponectin concentration with insulin resistance (IR) in a cross-sectional sample of 98 middle-aged premenopausal women with a wide range of obesity. In addition, we studied the relationship between adiponectin, body composition, and blood biochemical and cardiorespiratory fitness variables. Body composition and fat distribution were measured via dual-energy x-ray absorptiometry in normal-weight (NW) (n = 41, body mass index [BMI] < 25 kg/m(2)) and overweight (OW) (n = 57, BMI > or = 25 kg/m(2)) women. Fasting blood samples were obtained; adiponectin, leptin, insulin, glucose, and insulin-like growth factor-I were measured; and IR index was calculated. The IR index from fasting plasma insulin and plasma glucose levels was estimated using the homeostasis model assessment (HOMA), as follows: fasting plasma insulin (in microliter units per milliliter) x fasting plasma glucose (in millimoles per liter)/22.5. Adiponectin was significantly higher (P = .0001) in NW (14.7 +/- 4.7 microg/mL) compared with OW (9.9 +/- 3.1 microg/mL) women. Significant differences (P < .003) in body mass, BMI, percentage of fat mass, fat mass, trunk fat, trunk fat-leg fat ratio, leptin, insulin, and HOMA were also observed between NW and OW groups. Leptin was independently related to plasma adiponectin (beta = -.259, P = .001) in the overall study group. Plasma adiponectin was only related to trunk fat-leg fat ratio (beta = -.242, P = .002) among NW subjects, whereas plasma adiponectin was related to fat-free mass (beta = .182, P = .0001) and HOMA (beta = -.576, P = .002) among OW women. The inverse relationship between adiponectin and leptin concentrations suggests that leptin may be involved in the regulation of adiponectin in middle-aged premenopausal women. Our data also demonstrate that adiponectin may play an important role in sustaining insulin sensitivity only in OW middle-aged premenopausal women.