1. Serum cholesterol reduction and changes in cholesterol metabolism were studied during rapeseed oil feeding without and with increasing amounts of sitostanol trans-esterified with rapeseed oil fatty acids and dissolved in rapeseed oil mayonnaise. Fifteen mildly hypercholesterolaemic subjects replaced 50 g of their usual dietary fat by 50 g of rapeseed oil fat mayonnaise for 6 weeks followed by randomization so that eight subjects continued on rapeseed oil mayonnaise alone (control group) for 15 weeks and seven on rapeseed oil mayonnaise with a small dose of sitostanol ester (800 mg/day of sitostanol) for 9 weeks followed by 6 weeks with higher dose of sitostanol ester (2000 mg/day of sitostanol). 2. During the rapeseed oil period the reduction in serum low-density lipoprotein cholesterol was 14% from the home diet. The control-adjusted reduction by the low sitostanol ester dose was 7.4% (not significant) and by the higher dose it was 15.7%. 3. The low dose of sitostanol ester had no consistent effect on cholesterol precursors or cholestanol in serum, reduced serum levels of campesterol and sitosterol by 28.2% and 23.6%, respectively, and reduced cholesterol absorption efficiency significantly from 28.7% to 23.4%. In accordance, faecal excretion of neutral and particularly endogenous neutral sterols increased (16.7% and 19.7%, respectively), but faecal cholesterol elimination and cholesterol synthesis were only insignificantly increased. 4. During the high dose of sitostanol ester the high-density lipoprotein-to low-density lipoprotein-cholesterol ratio increased.(ABSTRACT TRUNCATED AT 250 WORDS)