The objective of this study was to evaluate whether plant sterol-ester margarine has an additive or interactive effect on low-density lipoprotein (LDL) cholesterol reduction when ingested in combination with a statin drug. This was a multicenter, randomized, double-blind study with 4 parallel treatment arms in a balanced 2 x 2 factorial design. The 4 daily treatment options were: (1) placebo plus regular margarine 25 g (n = 38); (2) placebo plus sterol-ester margarine 25 g (2 g of plant sterol) (n = 39); (3) cerivastatin 400 microg plus regular margarine 25 g (n = 38); and (4) cerivastatin 400 microg plus sterol-ester margarine 25 g (n = 37). The study was conducted in men and women with primary hypercholesterolemia with baseline LDL cholesterol >/=97 mg/dl (mean 206). The primary efficacy parameter was the percent change in LDL cholesterol between baseline and at the end of 4 weeks' treatment. Cerivastatin (vs placebo) reduced LDL cholesterol by 32% (95% confidence intervals 28% to 36%, p <0.0001) and sterol-ester margarine (vs regular margarine) by 8% (95% confidence interval 4% to 12%, p <0.0001). The effect of sterol-ester margarine and cerivastatin together was additive (39% reduction in LDL cholesterol), but there was no significant interactive effect between sterol-ester margarine and cerivastatin (p = 0.29). The treatments were generally well tolerated with no major differences in adverse events between groups. In subjects with primary hypercholesterolemia, statin and sterol-ester margarine used together produce a purely additive effect on LDL cholesterol reduction. The addition of sterol-ester margarine to statin therapy offers LDL cholesterol reduction equivalent to doubling the dose of statin.