OBJECTIVE: To determine whether the lipoprotein response to weight loss in obese patients with type 2 diabetes can be improved by modifying the macronutrient composition of the commonly prescribed low-fat, high-carbohydrate (CHO) hypocaloric diet. RESEARCH DESIGN AND METHODS: Nine obese patients with type 2 diabetes were treated with a monounsaturated fatty acid (MUFA)-enriched weight-reducing formula diet and compared with eight obese patients with type 2 diabetes treated with a low-fat, high-CHO weight-reducing formula diet. Weight loss ensued for 6 weeks, followed by 4 weeks of refeeding using isocaloric formulas enriched with MUFA or CHO, respectively. Fasting blood samples were obtained to measure plasma lipoproteins and LDL susceptibility to oxidation (measured as lag time: time required to induce in vitro formation of conjugated dienes). RESULTS: At baseline, there were no differences between the groups in plasma lipids, lipoproteins, or LDL susceptibility to oxidation. Weight loss was similar between the groups. Dieting resulted in decreases in total plasma cholesterol, LDL, HDL, triglycerides, and apolipoproteins A and B (P < 0.05), but the MUFA group manifested a greater decrease in total cholesterol, triglycerides, and apolipoprotein B and a smaller decrease in HDL and apolipoprotein A than the CHO group (P < 0.05). Improvements in these parameters were sustained during refeeding. After dieting, lag time was prolonged in the MUFA group (208 ± 10 min) compared with the CHO group (146 ± 11 min; P < 0.05). Lag time was prolonged further during refeeding in the MUFA group (221 ± 13 min, P = 0.10), while the CHO group remained unchanged (152 ± 9 min, P < 0.05). Lag time correlated strongly with the oleic acid content of LDL after dieting and refeeding (r = 0.74 and r = 0.93, respectively; both P < 0.001). CONCLUSIONS: Macronutrient content is an important determinant of the lipoprotein response to weight loss in obese patients with type 2 diabetes. MUFA-enriched hypocaloric diets potentiate the beneficial effects of weight loss to ameliorate cardiovascular risk factors in obese patients with type 2 diabetes.