BACKGROUND Icosapent ethyl (IPE; formerly AMR101) is a high-purity prescription form of eicosapentaenoic acid ethyl ester. In the MARINE study we evaluated the efficacy and safety of IPE in patients with very high triglycerides (TG; ≥500 mg/dL) and previously demonstrated significant reductions in TG levels with no significant increases in low-density lipoprotein (LDL) cholesterol levels. OBJECTIVE In this follow-up, exploratory analysis, we report the effects of IPE on lipoprotein particle concentration and size. METHODS MARINE was a phase 3, multicenter, placebo-controlled, randomized, double-blind, 12-week study. Hypertriglyceridemic patients (N = 229) were randomized to three treatment groups: IPE 4 g/day, IPE 2 g/day, or placebo. Lipoprotein particle concentrations and sizes were measured by nuclear magnetic resonance spectroscopy. RESULTS Compared with placebo, IPE 4 g/day significantly reduced median concentrations of large very-low-density lipoprotein (VLDL; -27.9%; P = .0211), total LDL (-16.3%; P = .0006), small LDL (-25.6%; P < .0001), and total high-density lipoprotein (HDL; -7.4%; P = .0063) particles and reduced VLDL particle size (-8.6%; P = .0017). In this patient population with TG ≥500 mg/dL, IPE did not significantly change the overall sizes of LDL or HDL particles. CONCLUSIONS IPE 4 g/day significantly reduced large VLDL, total LDL, small LDL, and total HDL particle concentrations and VLDL particle size in patients with TG ≥500 mg/dL. Changes in VLDL particle concentration and size reflect the TG-lowering effects of eicosapentaenoic acid. The reduction in LDL particle concentration with IPE is novel among ω-3 therapies and is consistent with the previously reported reduction in apolipoprotein B and lack of LDL-C increase with IPE in patients with very high TG levels. BACKGROUND NCT01047683.