CONTEXT: F2-isoprostanes (F2-isoPs) are biomarkers for oxidative stress in humans and have been shown to be elevated in obesity, cardiovascular disease, and diabetes. Therefore, F2-isoPs are often implicated in oxidative stress contributing to insulin resistance, although this has not been rigorously examined. OBJECTIVE: To determine whether urinary F2-isoPs are predictive of insulin sensitivity and other clinical metabolic parameters. PARTICIPANTS: Sedentary, weight-stable, nondiabetic adults equilibrated on a standard isocaloric diet. MAIN OUTCOME MEASURES: Insulin sensitivity via hyperinsulinemic-euglycemic clamp, urinary F2-isoPs by gas chromatography-mass spectrometry, and body composition by dual-energy x-ray absorptiometry. RESULTS: No correlation was found between 15-F2t-IsoP nor its major metabolite, 2,3-dinor-5,6-dihydro-15-F2t-IsoP, with insulin sensitivity, even after adjusting for age, race, sex, BMI, and smoking status. 15-F2t-IsoP was also not associated with body fat. However, there was a strong negative correlation between 15-F2t-IsoP and lean body mass (LBM; r = -0.46, P = 0.0001), bone mineral content (BMC; r = -0.58, P < 0.0001), bone mineral density (BMD; r = -0.65, P < 0.0001), and skeletal muscle protein 4-hydroxynonenal (4-HNE; r = -0.54, P = 0.0239), another marker of oxidative stress. 15-F2t-IsoP was also positively associated with circulating triglycerides and total cholesterol, and increased as a function of age. CONCLUSIONS: Urinary 15-F2t-IsoP and its major metabolite are not associated with insulin sensitivity, suggesting the lipid peroxidation process that produces F2-isoPs does not reflect oxidative stress reactions operative in insulin resistance. However, urinary F2-isoPs were negatively correlated with LBM, BMC, BMD, and muscle 4-HNE. Because lean and bone mass decline as a function of biological aging, F2-isoPs may reflect the oxidative stress operative in the aging process.