Disruption of one allele for the cytosine-DNA methyltransferase 1 (DNMT1) gene in mice with a germ-line mutation in a tumor suppressor gene was shown previously to reduce tumor formation in juvenile animals. This effect is now reproduced in our studies of mature mice where this genetic DNMT1 reduction leads to a 50% decrease in tobacco carcinogen-induced lung cancer and a similar reduction in DNMT activity in type II pneumocytes that give rise to the tumors. Short-term treatment of DNMT wild-type female mice with low doses of the demethylating agent 5-aza-2'-deoxycytidine decreased the incidence of neoplasms by 30%. Importantly, when 5-aza-2'-deoxycytidine was combined with the histone deacetylase inhibitor sodium phenylbutyrate, lung tumor development was significantly reduced by >50%; no effect was seen with phenylbutyrate alone. This identical combination of inhibitors also acts synergistically to cause re-expression of densely hypermethylated and transcriptionally silenced tumor suppressor genes in human cancer cells. Thus, reduction in DNMT and histone deacetylase activities that likely block epigenetically mediated gene silencing might provide a novel clinical strategy to help prevent the leading cause of cancer death in the United States.