Prescription omega-3 acid ethyl esters (P-OM3) are commonly used for treatment of very high triglyceride levels, often in combination with a statin, to lower persistent hypertriglyceridemia. This randomized, crossover trial evaluated 6 weeks of combination therapy with simvastatin 20 mg/day plus P-OM3 4 g/day or placebo in 39 men and women (average age 58 years) with a triglyceride concentration 200 to 600 mg/dl and non-high-density lipoprotein (non-HDL) cholesterol greater than their National Cholesterol Education Program treatment goals after a 5-week diet lead-in. Non-HDL cholesterol decreased from baseline (209 mg/dl) by 40% for P-OM3 + simvastatin compared with 34% for placebo + simvastatin (p <0.001). Favorable changes for P-OM3 + simvastatin versus placebo + simvastatin were also observed for very low-density lipoprotein (VLDL) cholesterol (-42% vs -22%), triglyceride (-44% vs -29%), total cholesterol (-31% vs -26%), HDL cholesterol (+16% vs +11%), apolipoprotein B (-32% vs -28%), total cholesterol:HDL cholesterol ratio (-39% vs -33%), triglyceride:HDL cholesterol ratio (-51% vs -37%), and systolic (-5.0 vs 0.3 mm Hg) and diastolic (-3.3 vs -1.8 mm Hg) blood pressures (p <0.05 for all). VLDL particle concentration and size decreased and LDL particle size increased significantly more with P-OM3 + simvastatin than with placebo + simvastatin (all p <0.05). Changes in LDL cholesterol, LDL particle concentration, HDL particle size and concentration, and apolipoprotein A-I did not differ significantly between treatments. In conclusion, P-OM3 + simvastatin appears to be a useful therapeutic option for the management of mixed dyslipidemia.