We examined the modifying effects of heated garlic (Allium sativum L.) on N-ethyl-N’-nitro-N-nitrosoguanidine (ENNG)-induced duodenal and jejunal carcinogenesis in mice. Heated garlic powder used in this study was prepared as follows: unpeeled garlic bulbs were blanched in boiling water for 6 min, and then peeled, the cloves being crushed, homogenized, and finally freeze-dried. The garlic powder had almost undetectable alliinase activity and was rich in alliin (the main sulfur compound of heated garlic; 22.1 mg/g dry weight). Male C57BL/6 mice were given ENNG (100 mg/l) in drinking water for the first 4 weeks, and then basal diet (Group 1), or 10% (Group 2), 3% (Group 3) or 1% (Group 4) heated garlic in the diet for 30 weeks. At the termination of the experiment, the incidences of duodenal tumors in Groups 1-3 were significantly lower than those in Group 1, and the multiplicities in Group 2 were significantly lower than those in Group 1. Additionally, the incidences and/or multiplicities of the jejunal tumors in Groups 2 and 4 were also significantly lower than those in Group 1. In this study, we also examined changes in erythrocyte polyamine levels. Values for Group 1 were significantly greater than those in the control group, and this elevation in Group 1 were significantly inhibited by dietary heated garlic (10% in the diet; Group 2). These results indicated that the post-initiation-stage feeding of heated garlic, especially at 10% in the diet, inhibits ENNG-induced duodenal and jejunal carcinogenesis in mice.