Six subjects with ileostomies consumed five diets containing 61-164 g starch/d of which 0.4-34.8 g was resistant starch (RS). Ileal excretion of starch was 97% of that measured as dietary RS in vitro with no significant difference between RS fed and starch recovered on any of the test diets. Variation in starch excretion between subjects was partly due to differences in mouth-to-stoma transit time. In vitro fermentation of ileal effluent from RS-supplemented diets produced significantly more short-chain fatty acids, a higher molar proportion of butyrate (17% compared with 12%), and a lower concentration of ammonia compared with control subjects. These results indicate that the amount of starch that reaches the large intestine can be predicted from measurements in vitro for a wide range of RS intakes under normal eating conditions. They also support the hypothesis that RS, through fermentation, has distinctive influences on the colonic environment.