A test of the anticancer effects of dietary fiber was conducted using the N-nitrosomethylurea (NMU)-induced rat mammary tumor model. Starting 3 days after NMU treatment, four different groups of F344 rats (30 rats in each group) were fed as follows: Group 1 received a high-fat diet; group 2, a high-fat plus fiber diet (soft white wheat bran, 10% wt/wt); group 3, a low-fat diet; and group 4, a low-fat plus fiber diet. The rats remained on these diets for 15 weeks. Tumor incidence in group 1 was 90% compared with 66% in group 2 (P less than .001). Tumor incidence in group 3 was 63% compared with 47% in group 4 (P greater than .4). These results show that supplemental dietary fiber exerts an inhibitory effect on the promotional phase of NMU-induced mammary carcinogenesis in rats when fed a high-fat but not a low-fat diet. To test whether fiber may exert its antipromoting effect by reducing circulating estrogens, serum 17 beta-estradiol was assayed. No changes were observed in serum 17 beta-estradiol levels among the four groups, suggesting that the protective effect of fiber in this animal model is not mediated by a fiber-induced reduction of circulating 17 beta-estradiol.