Y-700, 1-[3-cyano-4-(2,2-dimethylpropoxy)phenyl]-1H-pyrazole-4-carboxylic acid, is a newly synthesized inhibitor of xanthine oxidase. This study found that feeding of Y-700 suppressed the development of colonic aberrant crypt foci, precursor lesions of colon cancer, and cell proliferation in 1,2-dimethylhydrazine-treated mice, accompanied by reduced serum urate. These results suggest that Y-700 is a useful agent for the prevention of colon tumorigenesis and that xanthine oxidase plays an important role in the development of colon cancer.