Background: DNA methylation is an important mechanism for gene silencing and has already been described for several genes in breast cancer. A previous immunohistochemistry study demonstrated a decrease of K_v1.3 potassium channel expression in breast adenocarcinoma compared to normal breast tissue. Methods: Methyl-specific PCR (MSP), immunohistochemistry and RNA extraction were performed on breast adenocarcinoma. MSP and DNA extraction were also performed on one breast carcinoma cell line and on primary culture normal cells. Results: DNA methylation of K_v1.3 gene promoter was observed in 42.3% of samples (22/52). The methylated status was associated with poorly differentiated tumors (p=0.04) and younger patients (p=0.048). Decreased K_v1.3 expression was observed in grade III tumors, at both the mRNA and protein levels, while methylation increased in grade III tumors. Finally, K_v1.3 gene promoter was methylated in the MCF-7 breast carcinoma cell line while promoter methylation was absent in primary culture of normal breast cells (HMEpC). Conclusion: We report, for the first time, the methylation of the K_v1.3 gene promoter in breast adenocarcinoma. Our data suggest that DNA methylation is responsible for a decrease of K_v1.3 gene expression in breast adenocarcinoma and is associated with poorly differentiated tumors and younger patients.