Context: To meet the worldwide challenge of emerging diabetes, accessible and inexpensive tests to identify insulin resistance are needed. Objective: To evaluate the sensitivity and specificity of the product of fasting, we compared the triglycerides and glucose (TyG) index, a simple measure of insulin resistance, with the euglycemic-hyperinsulinemic clamp test. Design and Setting: We conducted a cross-sectional study of the general population and outpatients of the Internal Medicine Department at the Medical Unit of High Specialty of the Specialty Hospital at the West National Medical Center in Guadalajara, Mexico. Patients: Eleven nonobese healthy subjects, 34 obese normal glucose tolerance individuals, 22 subjects with prediabetes, and 32 diabetic patients participated in the study. Intervention: We performed a euglycemic-hyperinsulinemic clamp test. Main Outcome Measures: Sensitivity and specificity of the TyG index Ln(fasting triglycerides) (mg/dl) × fasting glucose (mg/dl)/2 were measured, as well as the area under the curve of the receiver operating characteristic scatter plot and the correlation between the TyG index and the total glucose metabolism (M) rates. Results: Pearson′s correlation coefficient between the TyG index and M rates was −0.681 (P < 0.005). Correlation between the TyG index and M rates was similar between men (−0.740) and women (−0.730), nonobese (−0.705) and obese (−0.710), and nondiabetic (−0.670) and diabetic (−0.690) individuals. The best value of the TyG index for diagnosis of insulin resistance was 4.68, which showed the highest sensitivity (96.5%) and specificity (85.0%; area under the curve + 0.858). Conclusions: The TyG index has high sensitivity and specificity, suggesting that it could be useful for identification of subjects with decreased insulin sensitivity. The product of triglycerides and glucose closely mirrors the glucose clamp technique in the assessment of insulin sensitivity.