OBJECTIVE: To determine the effectiveness of prescribing 2 g plant sterols/stanols per day as an addition to standard practice in a dietary outpatient clinic. DESIGN: A randomized parallel design of comparative 12-week interventions. SUBJECTS/SETTING: Patients referred by a general practitioner to a dietary outpatient clinic for the management of hyperlipidemia were eligible. Twenty-five patients (15 women and 10 men) completed the study. INTERVENTION: Counseling regarding diet for hyperlipidemia was based on the National Cholesterol Education Program guidelines. The intervention group was instructed to incorporate approximately 25 g/day of margarine, containing plant sterols/stanols, which delivered approximately 2 g plant sterols/stanols. MAIN OUTCOME MEASURE: Changes in diet, body weight, and serum total cholesterol, low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, and triglycerides were measured. STATISTICAL ANALYSIS PERFORMED: Changes in dietary and biochemical outcomes were assessed using Student's t test. For nonnormally distributed data, Wilcoxon signed rank test was used, and Mann-Whitney U tests were conducted to determine the proportion of subjects reaching defined goals. The number needed to treat index was used to report effectiveness of the intervention. RESULTS: Five of 14 subjects in the intervention group compared with 0 of 11 in the control group achieved a reduction in serum cholesterol of >/=15% ( P <.05). Using the number needed to treat index, for each 2.8 patients counseled with routine prescription of plant sterols/stanols, one additional patient would obtain a reduction in cholesterol by >/=15% compared with conventional management. This was achieved without any detrimental effects on the dietary fatty acid profile. CONCLUSIONS: Routine prescription of margarine containing plant sterol/stanol is an effective strategy in the management of hypercholesterolemic patients in the clinical setting.