Potassium channel activity is required for cell cycle progression and altered expression and activity has been implicated in a number of carcinomas. One family of channels the two-pore domain potassium (K2P) channels are of particular interest since when expressed on the cell surface these channels are constitutively active. Altered expression of acid-sensitive K2P channels has been identified in a number of carcinomas and their expression is thought to be beneficial to cancers under conditions such as hypoxia or low glucose. To examine the expression of acid-sensitive K2P channels (K2P3.1, K2P9.1, K2P15.1) in cancer cell lines a combined approach using reverse transcriptase PCR, immunofluorescence and Western blotting was used. Expression of these channels was also assessed in 10 oesophageal carcinoma samples by immunohistochemistry. Proliferation (MTS) assays were performed to assess proliferation in K2P positive cancer cell lines when exposed to specific K2P channel inhibitors (genistein, methanandamide and ruthenium red). In addition, whole cell currents of cancer cell lines were analysed and K2P channels were found to contribute to these currents. Expression of K2P3.1 and K2P9.1 mRNA and protein expression was identified in several human cancer cell lines including breast MCF7, colorectal HCT116, SW480, SW620, lung A549 and oesophageal OE19. Also K2P9.1 positivity was found in a significant proportion, >30 % of oesophageal carcinoma tissue. Proliferation was significantly decreased in response to K2P inhibitors for colorectal but not for breast or lung cancer cell lines. SW480 proliferation was reduced by 46±4 % p≤0.05 (100µM genistein), 53±6 % p≤0.05 (50µM methanandamide) and 72±10 % p≤0.05 (100μM ruthenium red). The other colorectal cell lines showed similar reductions HCT116 57±4 % p≤0.01 (50µM methanandamide) and SW620 59±5 % p≤0.01 (50µM methanandamide), 76±4 % p≤0.05 (100μM ruthenium red). Cell proliferation was also reduced >40 % by a broad spectrum potassium channel blocker 4-AP (5mM). K2P channels alter cell resting membrane potential therefore increased expression in cancer cells would change this potential and could lead to activation of other potassium channels causing progression in the cell cycle. Acid-sensitive K2P channel are modulated by cell stress conditions (pH, hypoxia) and the physiological relevance of this in a cancer environment needs to be investigated. These findings provide evidence for a role of acid-sensitive K2P channels in cancer cell proliferation and identify K2P channels as a potential therapeutic target in cancer.