Down-regulation of the E-cadherin-mediated cell adhesion system is strongly related to cancer invasion and metastasis. Aberrant CpG hypermethylation in the promoter region of the E-cadherin gene has been shown to be responsible for reduction of E-cadherin expression. The present study was designed to test the hypothesis that the demethylating agent 5-aza-2'-deoxycytidine (AZA) can restore the E-cadherin system and reduce the potential for metastasis. AZA treatment modified the methylation status of the 5' CpG island in the E-cadherin promoter, and induced re-expression of E-cadherin in human cancer cells whose E-cadherin expression had been silenced. The re-expressed E-cadherin was correlated with increased in vitro aggregation and reduced motility. After inoculation of cancer cells (MDA-MB-435S) into the mammary fat pads of mice with severe combined immunodeficiency, the mice were treated for nine consecutive weeks with AZA three times per week i.p. The AZA treatment suppressed both growth of the primary tumor and lung metastasis in comparison with untreated controls, suggesting that the suppression of metastasis may be, at least partly, attributable to restoration of E-cadherin expression. Therefore, inhibition of DNA methylation may be useful for preventing cancer metastasis.