BACKGROUND: Olestra is a nonabsorbable fat substitute that consists of fatty acids esterified to a sucrose molecule. OBJECTIVE: To determine the effect of olestra consumption on measurements of fecal fat excretion. DESIGN: Controlled cross-over trial. SETTING: Clinical research center and outpatient research laboratory. PARTICIPANTS: 10 healthy volunteers. INTERVENTION: On days 1 to 6 of the study, participants consumed 5 oz of conventional potato chips per day; on days 7 to 12, they consumed 5 oz of potato chips containing 40 g of olestra per day. MEASUREMENTS: Quantitative measurement of fecal fat by the van de Kamer titration, van de Kamer gravimetric, and Jeejeebhoy gravimetric methods and qualitative assessment of fecal fat by Sudan III staining. RESULTS: Excellent correlation was seen among the three quantitative assays, but the van de Kamer titration method yielded lower measurements than the two gravimetric methods. When participants consumed 40 g of olestra per day, the excretion of fecal fat increased to levels observed in patients with steatorrhea caused by the malabsorption syndrome. CONCLUSION: Consumption of olestra can cause false-positive results on tests for steatorrhea and may therefore lead to an erroneous diagnosis of the malabsorption syndrome.