We compared the effects of ad libitum consumption of a defined high complex carbohydrate (CHO) diet (% of energy: CHO, 58.3; fat, 25.8) vs. a defined high monounsaturated fatty acid (MUFA) diet (% of energy: CHO, 44.7; fat, 40.1; MUFA, 22.5) on LDL electrophoretic characteristics. Healthy men [n = 65; age, 37.5 +/- 11.2 (mean +/- SD) y; BMI, 29.2 +/- 4.9 kg/m2] were randomly assigned to one of the two diets that they consumed for 6-7 wk. The high CHO diet significantly reduced body weight (-2%). The diet-induced reduction in plasma LDL cholesterol (C) levels in the high-CHO diet group was due mainly to concurrent reductions in the cholesterol content of small (<25.5 nm, P < 0.01) and medium-sized LDL (25.5-26.0 nm, P = 0.01). The high MUFA diet also reduced body weight, and LDL-C and LDL-apolipoprotein (apo)B levels, which were comparable to those in the high CHO group. The cholesterol levels of small LDL particles tended to be reduced (P = 0.24) in the high MUFA group (-12%), similar to changes in the high CHO group. These results suggest that, when associated with weight loss, ad libitum consumption of high CHO and high MUFA diets may be considered to be equally beneficial for the management of LDL-related atherogenic dyslipidemia. However, the high MUFA diet more favorably affected triglyceride levels, suggesting that it may be preferable to a high CHO diet in cardiovascular disease prevention.