Iron-enriched diets caused an increase of tumor rate in two models of dimethylhydrazine(DMH)-induced colon tumorigenesis in mice. The effect was independent of the time the iron-diet was fed, i.e. during DMH-treatment or following the DMH-treatment period. The increase of tumor rate depended on the iron concentration in the diet (0.5-3.5%). The concentration-dependent iron accumulation in the colonic mucosa of mice was paralleled by increments of malonaldehyde contents indicating lipid peroxidation, another factor known to be involved in tumor development. It is suggested that iron exerts cocarcinogenic activity in the DMH-model by stimulating cell proliferation and inducing oxidative stress in the colonic mucosa. This effect of iron is independent of the time of tumor-initiation by DMH, as it is also observed in the period of tumor-promotion/progression after DMH-treatment.