Potassium (K+) channels contribute to the regulation of cell proliferation and apoptosis and are also involved in tumor generation and malignant growth. Using immunohistochemical analysis, we investigated the expression of four K+ channels GIRK1 (G-Protein Inwardly Rectifying Potassium Channel 1), Ca2+-activated K channel (K Ca 1.1), voltage activated K+ channels (KV 1.1 and KV 1.3) and of the anti-apoptotic protein Bcl2 in normal and cancerous breast tissues and compared their expression with clinicopathological data. GIRK1 was overexpressed in carcinomatous tissues. In contrast, K V 1.1 and K V 1.3 were less expressed in cancerous tissue. The expression of Bcl-2 was similar in both tissues. As to the clinicopathological data, a correlation between K Ca 1.1 channel and estrogen receptor (ER) expression was observed. GIRK1 was overexpressed in breast carcinoma suggesting its involvement in proliferation and oncogenesis and its possible use as a putative pharmaceutical target. The correlation between K Ca 1.1 channel and ER suggests the involvement of this channel in proliferation. The loss of expression of the two channels K V 1.1 and K V 1.3 may correspond to their role in apoptosis.