OBJECTIVE It was hypothesized that resistin links obesity with diabetes, but this has not been studied in children and adolescents to date. METHODS We determined serum resistin levels of 135 obese (body mass index, 32.0 +/- 6.2 kg/m2; age, 12.6 +/- 3.4 yr) and 201 lean children (body mass index, 18.7 +/- 2.4 kg/m2; age, 12.5 +/- 2.5 yr) by a newly developed and extensively evaluated in-house immunoassay. These results were controlled for their association with markers of puberty, obesity, and insulin sensitivity. RESULTS The analytical evaluation of our assay revealed different resistin isoforms with major peaks of higher than 660 and 55 kDa in the size exclusion chromatography. Using this assay system we found no difference in the resistin levels of obese compared with lean subjects (P = 0.48). However, resistin was significantly higher in girls than in boys (6.74 +/- 2.42 vs. 5.79 +/- 2.45; P < 0.001). Interestingly, in both obese and lean children, resistin correlated with age (P < 0.01), Tanner stage, and testosterone and estradiol levels (P < 0.05). In contrast, no significant correlation was found with parameters of insulin resistance such as homeostasis model assessment, insulin sensitivity index, or insulin, proinsulin, and glucose concentrations in obese subjects. CONCLUSIONS Resistin appears to be not the main link between obesity and insulin resistance in children and adolescents but because of its association with Tanner stage, it may be related to the maturation of children during pubertal development. Additionally, we have demonstrated the presence of different molecular isoforms of resistin in human blood, and this may raise problems in comparing data from diverse assay systems.