OBJECTIVE: With increasing emphasis on the recognition of the metabolic syndrome and early type 2 diabetes, a clinically useful measure of insulin resistance is desirable. The purpose of this study was to evaluate whether an index of glucose metabolism, as measured by (13)CO(2) generation from ingested [(13)C]glucose, would correlate with indexes from the hyperinsulinemic-euglycemic clamp. RESEARCH DESIGN AND METHODS: A total of 26 subjects with varying degrees of insulin sensitivity underwent both the [(13)C]glucose breath test and the hyperinsulinemic-euglycemic clamp. Results from the [(13)C]glucose breath test were compared with measures of insulin sensitivity from the glucose clamp as well as with other commonly used indexes of insulin sensitivity. RESULTS: There was a strong correlation between the [(13)C]glucose breath test result and the glucose disposal rate (r = 0.69, P < 0.0001) and insulin sensitivity index (r = 0.69, P < 0.0001) from the insulin clamp. The magnitude of these correlations compared favorably with QUICKI and were superior to the homeostasis model assessment. CONCLUSIONS: The [(13)C]glucose breath test may provide a useful noninvasive assessment of insulin sensitivity.