Objective: To describe the design of the Olestra Postmarketing Surveillance Study (OPMSS) and to present baseline results from the sentinel site. The purpose of the OPMSS is to monitor consumption patterns of olestra- containing snacks and to determine whether consumption affects nutritional status. Design: The OPMSS combines repeated cross-section, random-digit dial telephone surveys before and after the market release of olestra-containing foods as well as intensive dietary and clinical assessments on a subsample of survey participants. Subjects: Data are from baseline telephone (n=1,962) and clinical (n=1,069) assessment of participants (aged 18 to 74 years) in the Marion County, Indiana, sentinel site. Mean age of participants in the telephone survey was 43.2 years; 19% of respondents were black and 29% had completed college. Statistical analysis: Analyses examined associations among savory snack use, fruit and vegetable consumption, and demographic and health-related characteristics. Data from the telephone survey were adjusted to be representative of the Marion County population. Results: Almost 96% of the population surveyed had eaten savory snacks in the month before the survey: 74% had eaten regular-fat, 26% fat-reduced, and 78% nonfat types. Total snack consumption did not differ by gender, education, or race. Residents younger than 35 years ate snacks 16 times a month compared with 12 times a month among older residents. Types of snacks consumed differed markedly by demographic characteristics. Male, younger, and less educated residents ate more regular-fat snacks; female, white, and college-educated residents ate more nonfat snacks. In general, residents practicing healthful behaviors, including not smoking, eating fruits and vegetables, and exercising, also ate fewer regular-fat and more nonfat snacks. Fat intake was also related linearly to use of snack foods, ranging from 33.2% of energy among those consuming 1 serving per month or less to 36.8% among those consuming 20 or more servings per month. Applications/conclusions: Procedures for recruitment and nutrition assessment appear adequate for evaluating the impact of olestra consumption on nutritional status. Nutritionists should be aware that there is potential for relatively high olestra consumption, given that almost 35% of Marion County residents eat snack foods at least 20 times a month. Consumers eating at least 20 servings of snacks per month derived more than 12% of their total energy and fat from snack foods, which suggests that substituting olestra snacks could substantially reduce intakes of fat and energy.