Objective: We investigated the serum phytosterol responses of heterozygous relatives of sitosterolemia patients to diets enriched in phytosterols or stanols. Design: Randomized double-blind crossover design. Setting: Muenster, Germany. Subjects: Eight heterozygous and 13 control subjects were recruited. One heterozygote and three controls dropped out. Interventions: Seven heterozygotes and 10 controls received daily portions of margarine containing 2 g of plant sterols, 2 g of stanols or a control margarine for 6 weeks each in a randomized order. These phases were intercepted by wash-out periods of 6 weeks each. Results: Compared to the control period, serum phytosterol concentrations increased overall by more than 20% when subjects consumed the plant sterol margarine (F (1,15) ¼ 8.719, P ¼ 0.01), with no significant difference between heterozygotes (mean þ 14.5 (s.d. 17.2) mmol/l, þ 23.0%) and controls ( þ 4.9 (9.9) mmol/l, þ 20.5%; F (1,15) ¼ 2.168, P ¼ 0.162), but decreased when subjects consumed the stanol-enriched margarine (F (1,15) ¼ 12.124, P ¼ 0.003), again to a similar extent in heterozygotes (À34.2 (41.2) mmol/l, À54.2%) and controls (À12.2 (9.2) mmol/l, À50.6%; F (1,15) ¼ 2.729, P ¼ 0.119). The lowest total serum concentrations of cholesterol and phytosterols were seen after the diet enriched in stanols. Serum stanol concentrations increased on this diet, but on a very low level and never exceeded 0.05% of serum cholesterol levels in any subject. Conclusions: Serum phytosterol concentrations increased only moderately in heterozygotes consuming a diet enriched in phytosterols, indicating that they retained considerable capacity to excrete phytosterols even at higher intakes. Sponsorship: Supported by a grant of the Stifterverband für die Deutsche Wissenschaft (project number TS022/12372/2002) to MK.