The carcinogenicity of young bracken (Pteridium aquilinum) used as a human food was studied in inbred strain ACI rats. Experimental groups were divided into 3 groups: Group I received for 4 months the pellets containing unprocessed bracken; Group II received for 4 months the pellets containing processed bracken that had been treated with boiling water; and Group III received for 18–27 days the extracted water of bracken. A control group was fed a normal diet. All Group I rats surviving beyond 7 months after the start of feeding developed tumors in the ileal regions. These tumors were not only epithelial, such as adenoma and adenocarcinoma, but also sarcomas. Cecal tumors were also induced. In Group II, the latent period of the tumor was longer than that in Group I. Furthermore, the incidence of intestinal tumors was lower than that in Group I, and sarcomas were not observed. However, the incidence of urinary bladder tumors was higher than that in the control group. Compared with the control group, Group III rats had no significant difference in the incidence of tumor. These results suggest that carcinogen in bracken can be removed to some extent by treatment with boiling water and that administration of a relatively small amount of bracken carcinogen for a long period results in a high incidence of tumors of the urinary bladder and the intestine.