Earlier studies (Kashket et al., 1991) showed that particles of high-starch snack foods remained longer on the teeth than those of high-sucrose, low-starch foods. The question arose whether the prolonged presence of food particles enhances cariogenicity. A study was undertaken to measure sugars, starches, and metabolic acids in retained food particles. Subjects consumed portions of different foods, and particles were removed from all bicuspids and first molars at defined times after swallowing. Dry weights, sugars, and short-chain carboxylic acids were determined. High-sucrose foods were cleared rapidly from the teeth, while high-starch foods were retained for up to 20 min. Sucrose, glucose, and fructose persisted in the retained particles. Particles of high-starch foods accumulated maltose and maltotriose, presumably from the breakdown of starch by salivary amylase. At maximum, maltose plus maltotriose constituted 94% of total sugars in particles of potato chips; corresponding values in doughnuts, peanut butter cookies, and salted crackers were 43, 51, and 61%, respectively. Total fermentable sugars in the particles of high-starch foods were similar to those for the high-sucrose confectionery products. Carboxylic acids accumulated within the particles, presumably due to the fermentation of the sugars by entrapped salivary micro-organisms. At maximum (5 to 7 min), acetic, formic, lactic, and propionic acids rose 17-, 30-, 15-, and 1.3-fold, respectively, in doughnuts, and to smaller degrees in potato chips, salted crackers, and chocolate-caramel-peanut bars. In summary, the study demonstrated the persistence of sugars, the progressive accumulation of starch breakdown products, and the fermentation of the accumulated sugars in retained food particles. The findings support the view that high-starch foods contribute to the development of caries lesions.