text: This study was designed to investigate the optimal dose of garlic during long-term feeding and its preventive and therapeutic effects on colon cancer in rats induced by 1,2-dimethylhydrazine (DMH). A total of 240 male Sprague-Dawley rats were grouped and fed with either a basal or a garlic diet of different concentration, and some groups were subcutaneously injected with DMH 20 mg/kg once a week for 20 weeks. The incidence of colon tumor was significantly decreased in the groups fed with 2.5%, 5%, and 10% garlic diets (p < 0.001). There was no distinct difference among these concentrations (p > 0.05). Therefore the minimal optimal dose of garlic to inhibit colon cancer was 2.5%. The equivalent dose of this concentration in humans is 4.76 g/m2 body surface/day. In a therapeutic study, the tumor-inducing interval in nude mice subcutaneously injected with colon cancer cells (CC-M2) was prolonged by a 2.5% garlic diet (p < 0.01). Thus smaller tumor volume and longer survival time were found in the garlic group than in the controls (p < 0.01). However, the growth rate of tumors was not markedly inhibited by garlic. All rats finally died within 18 weeks. This study suggested that a 2.5% garlic dose may be used mainly as an inhibitor to prevent colon cancers and improve survival time.