Insulin resistance is a strong predictor of the development of type 2 diabetes mellitus and cardiovascular disease. Girls with premature adrenarche (PA) or obesity may be at an increased risk for the development of insulin resistance. Recently, in prepubertal girls with PA, a fasting glucose to insulin ratio (FGIR) of less than 7 was found to be predictive of insulin resistance as determined by the frequently sampled iv glucose tolerance test. We sought to compare the FGIR with 2 insulin sensitivity measures, SiM (an adjusted mean measure of insulin sensitivity based on fasting and 2 h post glucose load insulin sensitivity measures) and the composite whole body insulin sensitivity index, ISI(comp), both derived from the 2-h oral glucose tolerance test in 2 groups of children at risk: girls with PA and obese girls. We studied 25 prepubertal girls with PA and/or obesity and further classified them as insulin resistant (IR) or insulin sensitive (IS) based on the FGIR. Four simple measures of insulin sensitivity [FGIR, quantitative insulin sensitivity check index (QUICKI), fasting insulin resistance index, and fasting insulin] were compared with SiM and ISI(comp). Additionally, we characterized the subjects in terms of risk factors associated with insulin resistance according to their insulin resistance status based on the FGIR. In our subjects the strongest correlations overall appeared to be between FGIR and SiM, FGIR and ISI(comp), QUICKI and SiM, and QUICKI and ISI(comp) [correlations (r) ranged from 0.81--0.84]. Furthermore, the IR group had higher body mass index and body mass index z-scores and triglyceride levels than the IS group and were over 3 times more likely to have triglycerides greater than the 95th percentile compared with national norms. We conclude that the FGIR and QUICKI are highly correlated with oral glucose tolerance test measures of insulin sensitivity. An FGIR less than 7 in young girls with PA or obesity may be helpful in the early identification of children at risk for complications of insulin resistance.