Objective— To assess the effect of a Mediterranean diet (MedDiet) with and without weight loss (WL) on apolipoprotein B 100 (apoB 100 ) metabolism in men with metabolic syndrome. Approach and Results— The diet of 19 men with metabolic syndrome (age, 24–62 years) was first standardized to a North American isoenergetic control diet for 5 weeks, followed by an isoenergetic MedDiet for an additional 5 weeks under full-feeding conditions (MedDiet−WL). Participants next underwent a 20-week supervised WL program under free-living conditions (−10.2±2.9% body weight; P <0.01) and finally consumed the MedDiet (5 weeks) under weight-stabilizing feeding conditions (MedDiet+WL). In vivo kinetic of apoB 100 was assessed in the fasted state at the end of the 3 controlled diets using a bolus of D 3 -leucine. Compared with the control diet, MedDiet−WL reduced low-density lipoprotein (LDL)-apoB 100 pool size (−14.2%, P <0.01) primarily through an increase in LDL-apoB 100 fractional catabolic rate (+30.4%, P =0.02) and increased LDL particle size ( P <0.01) but had no effect on very-LDL (VLDL)-apoB 100 pool size or triglyceride concentrations, despite a significant increase in VLDL-apoB 100 fractional catabolic rate (+25.6%; P =0.03). MedDiet+WL had no further effect on LDL-apoB 100 pool size and fractional catabolic rate but further increased LDL particle size and reduced VLDL-apoB 100 pool size versus the control diet primarily through an increase in VLDL-apoB 100 fractional catabolic rate (+30.7%; P <0.01). Conclusions— Consumption of MedDiet increases LDL size and reduces LDL-apoB 100 concentrations primarily by increasing the catabolism of LDL even in the absence of WL in men with metabolic syndrome. MedDiet seems to have a trivial effect on VLDL concentrations and kinetics unless accompanied by significant WL. Clinical Trial Registration —URL: http://www.clinicaltrials.gov . Unique identifier: NCT00988650.