BACKGROUND: Prebiotics are nondigestible food ingredients that stimulate the growth of Bifidobacterium and other bacteria in the gastrointestinal tract. Improved gastrointestinal and other health effects have been attributed to them. OBJECTIVE: The objective of this study was to evaluate the effects of dietary supplementation with the prebiotic oligofructose with and without zinc on the prevalence of diarrhea in a community with a high burden of gastrointestinal and other infections. DESIGN: Two consecutive randomized, blinded, controlled clinical trials were performed in a shantytown community near Lima, Peru. The first trial compared an infant cereal supplemented with oligofructose (0.55 g/15 g cereal) with nonsupplemented cereal. During the second trial, zinc (1 mg/15 g cereal) was added to both oligofructose-supplemented and control cereals. RESULTS: We enrolled 282 infants in the first trial and 349 in the second. In the first trial, mean (+/- SD) days of diarrhea were 10.3 +/- 9.6 in the nonsupplemented cereal group and 9.8 +/- 11.0 in the prebiotic-supplemented cereal group (P = 0.66). In the second trial, mean days of diarrhea were 10.3 +/- 8.9 in the group consuming cereal fortified only with zinc and 9.5 +/- 8.9 in the group consuming cereal containing both zinc and prebiotics (P = 0.35). Postimmunization titers of antibody to Haemophilus influenzae type B were similar in all groups, as were gains in height, visits to clinic, hospitalizations, and use of antibiotics. CONCLUSIONS: Cereal supplemented with prebiotics was not associated with any change in diarrhea prevalence, use of health care resources, or response to H. influenzae type B immunization. Infants and young children who continue to breast-feed may not benefit from prebiotic supplementation.