BACKGROUND: omega-3 fatty acids are known to improve lipid profiles, the distribution of lipoprotein subclasses, and secondary prevention against post-myocardial infarction. Rare reports have emerged of synergistic results of omega-3 fatty acids with simvastatin in cases of type 2 diabetes mellitus with hypertriglyceridemia. The purpose of this study was to determine the combined relationship of omega-3 fatty acids plus simvastatin on lipid, lipoprotein size and the types of subspecies. METHODS: This randomized, multi-center, comparison study evaluated eight weeks of combination therapy (omega-3 fatty acids (Omacor) 4 g/day plus simvastatin 20 mg/day) or monotherapy (simvastatin 20 mg/day) for at least six weeks in 62 diabetic patients. Subjects with a triglyceride concentration of more than 200 mg/dL were eligible for inclusion. RESULTS: No significant differences for omega-3 fatty acids + simvastatin versus simvastatin alone were observed for triglycerides (-22.7% vs. -14.3%, P = 0.292), HDL peak particle size (+2.8% vs. -0.4%, P = 0.076), LDL mean particle size (+0.4% vs -0.1%, P = 0.376) or LDL subspecies types, although the combination therapy showed a tendency toward lower triglycerides, larger HDL, and LDL particle sizes than did the monotherapy. There were no significant differences between the two groups in regard to HDL-C, LDL-C, or HbA1c levels. There were no serious adverse events and no abnormalities in the laboratory values associated with this study. CONCLUSION: omega-3 fatty acids were a safeform of treatment in hypertriglyceridemic patients with type 2 diabetes mellitus. But, regarding efficacy, a much larger sample size and longer-term follow-up may be needed to distinguish between the effects of combination therapy and monotherapy.