This study was designed to determine the effect of 10% dietary long-chain inulin on the azoxymethane (AOM)-induced colonic preneoplastic aberrant crypt foci (ACF) and small intestinal and colon tumors at the initiation (I), promotion (P) and I + P stages (20 rats per treatment) in Fisher 344 male weanling rats. After an acclimatization period of 1 wk, groups of Fisher 344 male weanling rats were assigned to consume AIN 93G diet (control) or AIN 93G diet containing 10% inulin. All the rats received 16 mg/kg body AOM dissolved in saline subcutaneously at 7 wk of age followed by a second injection at 8 wk of age. An additional group of five rats received only saline and consumed the control diet. The rats received the assigned diets until asphyxiation by CO(2) at 16 wk of age for the ACF experiment and 45 wk for the end-point tumor experiment. Feed intake, weight gain, diarrheal index, cecal weight, cecal pH, ACF and tumors in the colon were determined. Rats fed inulin had diarrhea after 2 wk of feeding and recovered by approximately 4 wk. Cecal weight was greater in rats fed inulin and cecal pH was lower. The inulin group had more than 66% fewer aberrant crypts and 60% fewer ACF compared with the control group. Tumor incidences in the small intestine and colon of rats in the control, I, P and I + P groups were: 78, 31, 0 and 11% and 90, 73, 69 and 50%, respectively. The corresponding values for the distal portion of the colon were 87, 63, 45 and 33%, respectively. Colon tumors per tumor-bearing rat were 4.2, 3.09, 1.36 and 1.2 for the control, I, P and I + P groups, respectively. All groups differed, P < 0.05. The results of this study indicate that dietary long-chain inulin suppresses AOM-induced ACF formation, an early preneoplastic marker of colon tumorigenesis in rats, and colon tumors, particularly at the promotion stage.