Adenosine triphosphate (ATP) is a ubiquitous extracellular messenger elevated in the tumor microenvironment. ATP regulates cell functions by acting on purinergic receptors (P2X and P2Y) and activating a series of intracellular signaling pathways. We examined ATP-induced Ca²⁺ signaling and its effects on anti-apoptotic (Bcl-2) and pro-apoptotic (Bax) proteins in normal human airway epithelial cells and lung cancer cells. Lung cancer cells exhibited two phases (transient and plateau phases) of increase in cytosolic [Ca²⁺] ([Ca²⁺]_cyt) caused by ATP, while only the transient phase was observed in normal cells. Removal of extracellular Ca²⁺ eliminated the plateau phase increase of [Ca²⁺]_cyt in lung cancer cells, indicating that the plateau phase of [Ca²⁺]_cyt increase is due to Ca²⁺ influx. The distribution of P2X (P2X1-7) and P2Y (P2Y₁, P2Y₂, P2Y₄, P2Y₆, P2Y₁₁) receptors was different between lung cancer cells and normal cells. Pro-apoptotic P2X₇ was nearly undetectable in lung cancer cells, which may explain why lung cancer cells showed decreased cytotoxicity when treated with high concentration of ATP. The Bcl-2/Bax ratio was increased in lung cancer cells following treatment with ATP; however, the anti-apoptotic protein Bcl-2 demonstrated more sensitivity to ATP than pro-apoptotic protein Bax. Decreasing extracellular Ca²⁺ or chelating intracellular Ca²⁺ with BAPTA-AM significantly inhibited ATP-induced increase in Bcl-2/Bax ratio, indicating that a rise in [Ca²⁺]_cyt through Ca²⁺ influx is the critical mediator for ATP-mediated increase in Bcl-2/Bax ratio. Therefore, despite high ATP levels in the tumor microenvironment, which would induce cell apoptosis in normal cells, the decreased P2X₇ and elevated Bcl-2/Bax ratio in lung cancer cells may enable tumor cells to survive. Increasing the Bcl-2/Bax ratio by exposure to high extracellular ATP may, therefore, be an important selective pressure promoting transformation and cancer progression.