Background: The purpose of this study was to evaluate the effect of highly purified eicosapentaenoic acid (EPA) in increasing low-density lipoprotein (LDL) particle size, as one of the possible mechanisms by which intake of EPA may prevent coronary events. Methods and Results: Hypertriglyceridemic subjects were randomly assigned to a control group (n=72) or an EPA group (n=72; EPA regimen 1,800mg/day for 6 months). In the EPA group, the serum LDL-cholesterol and high-density lipoprotein cholesterol levels remained unchanged, but there was a significant increase in LDL particle size based on LDL-relative mobility measured on lipoprotein polyacrylamide-gel electrophoresis, and a significant decrease in serum triglyceride-rich lipoproteins (TRLs) level. None of these changes were observed in the control group. After adjustments for coronary risk factors, multivariate logistic regression analysis identified elevation of serum EPA-related markers (6-month EPA, 6-month EPA/arachidonic acid [AA] ratio, change in [Δ] EPA, and EPA/AA), and treatment with statins and EPA as independent variables associated with increase in LDL particle size. Negative correlations were found between ΔTRLs and ΔLDL particle size, suggesting that improvement in triglyceride metabolism was associated with an increase in LDL particle size. Conclusions: EPA increases LDL particle size by improving triglyceride metabolism; and serum EPA level and EPA/AA ratio after EPA treatment may be useful markers of increased LDL particle size.  (Circ J 2013; 77: 2349–2357)