Insulin resistance and obesity are intimately related to a chronic low grade systemic inflammation. Interleukin-6 (IL-6) may influence the pathogenesis of obesity-related diseases. The aim of this study is to investigate the effect of body's fat mass on the relationships between -174G/C IL-6 promoter gene polymorphism, IL-6 circulating level and insulin resistance. A population of 150 Caucasian women was studied, subdivided according to their body composition in non-obese (NW), Normal Weight Obese (NWO) and preobese-obese (OB). The NWO subjects were found in an intermediate position between the NW and OB subjects in terms of body weight, fat mass percentage (FM%), abdominal FAT%, hs-CRP and plasma triglyceride level. Fasting plasma IL-6 concentration was positively correlated with the homeostasis model assessment for insulin resistance (HOMA-IR) in all subjects analyzed (P=0.0014). In NWO and OB women a significantly increased IL-6 mean value was observed compared with NW subjects. In G/G population, the IL-6 plasma level of NWO and OB was significantly higher with respect to NW. No significant differences of IL-6 concentrations were observed in the three groups carrying G/C genotype. NWO and OB women homozygous for the allele C have significantly lower value of IL-6 with respect to NW subjects. IL-6 concentration was positively correlated with FM% in G/G (R(2)=0.397, P<0.001) and was negatively correlated in C/C (R(2)=0.459, P=0.002). No significant correlation was observed in G/C genotype (R(2)=0.041, P=0.173). In conclusion our study confirms that, at least in Italian Caucasian females, the FM% is a major determinant of an increase in IL-6 production and insulin resistance. -174 G/C IL-6 promoter polymorphism represents a marker which could help to identify, time in advance, "vulnerable" individuals at risk of age and obesity related diseases.