The long-term cholesterol lowering effect of sitostanol, a 5-α saturated derivative of sitosterol solubilized in rapeseed oil margarine, was studied in a double-blind placebo-controlled random population sample (n=150) with mild hypercholesterolemia. At baseline all subjects replaced 24 g/d of their dietary fat by 3 8 g buttons of the rapeseed oil margarine for six weeks. At the end of this period the subjects were randomised to continue on the rapeseed oil margarine without (C; n=50) or with (S1; n=100) 3 g/d of sitostanol ester added to the margarine. After six months, S1 was rerandomized either to continue with 3 g/d (S1; n=50) or with 2 g/d (S2; n=50) of sitostanol. The study lasted for one year. Sitostanol ester was well-tolerated. In C, serum total, LDL or HDL cholesterol or triglyceride values did not consistently change. In S1, serum total and LDL cholesterol were reduced significantly by 11% and 15%, respectively, after one year of treatment. In S2, the respective significant reductions were 8% and 13%. HDL cholesterol and serum triglyceride values were unchanged by sitostanol, so that theHDL cholesterol ratio was markedly increased. Serum plant sterol and cholestanol proportions were decreased and those of cholesterol precursors increased in the S1 and S2 groups. After returning back to the home diet the values in S1 and S2 returned back to the initial levels. In conclusion sitostanol ester dissolved in rapeseed oil margarine lowered effectively serum total and LDL cholesterol level during one-year treatment without any side-effects.