Objective: To determine whether the lipid abnormalities observed in obese adolescents are associated with insulin resistance. Methods: We evaluated the relationship between lipid levels and insulin resistance in 82 obese adolescents. Insulin resistance was assessed by fasting insulin level and sum of the insulin values after an oral glucose tolerance test in all 82, and were compared with data from 40 nonobese adolescents. Whole-body glucose uptake during euglycemic hyperinsulinemia (M value) was performed in 19 of the obese adolescents and compared with that of 24 nonobese young adults. Results: The obese adolescents had significantly elevated low-density lipoprotein cholesterol (LDL-C) (3.09 ± 0.73 mmol/L; 119 ± 28.2 mg/dl) and triglycerides (1.22 ± 0.62 mmol/L; 108 ± 54.6 mg/dl) and low high-density lipoprotein cholesterol (HDL-C) levels (0.94 ± 0.24 mmol/L; 36 ± 9.1 mg/dl) when compared with values in the nonobese subjects. M values were significantly depressed in the obese compared with the nonobese subjects. Adiposity significantly correlated with low HDL-C and elevated triglyceride values. From the variables representing insulin resistance, the strongest correlation with the abnormal lipid profile was found for the M value. A stepwise multiple regression analysis revealed that the M value was the only step entered into the relationship for triglycerides and LDL-C, and both M value and fasting insulin were entered for HDL-C. Conclusion: In obese adolescents the degree of insulin resistance explains a significant portion of the variance in the levels of triglycerides, LDL-C, and HDL-C. (J P EDIATR 1995;126:690-5).