Oligofructose and inulin, naturally-occurring fermentable chicory fructans, have been shown to stimulate the growth of bifidobacteria which are regarded as beneficial strains in the colon and inhibit colon carcinogenesis in the laboratory animal models. The present study was designed to determine the effect of oligofructose and inulin on the azoxymethane (AOM)-induced preneoplastic lesions such as aberrant crypt foci (ACF) formation in the colon of male F344 rats. At 5 weeks of age, groups of animals were fed the AIN-76A (control) and the experimental diets containing 10% oligofructose or inulin. At 7 weeks of age, all animals received s.c. injection of AOM dissolved in normal saline at a dose rate of 15 mg/kg body wt, once weekly for 2 weeks. The animals were necropsied 7 weeks after the last AOM injection, and the ACF were visualized under light microscopy in the formalin-fixed, unsectioned methylene blue-stained colons. They were distinguished by their increased size, more prominent epithelial cells and pericryptal space. The feeding of oligofructose or inulin significantly inhibited the ACF formation and the crypt multiplicity in the colon. The degree of ACF inhibition was more pronounced in animals fed inulin than in those fed oligofructose. The findings suggest that chicory fructan supplements inhibit ACF formation, an early preneoplastic marker of malignant potential in the process of colon carcinogenesis.