TRPM2 is a Ca²⁺-permeable channel activated by oxidative stress or TNF, and TRPM2 activation confers susceptibility to cell death. The mechanisms were examined here in human monocytic U937-ecoR cells. This cell line expresses low levels of full length TRPM2 (TRPM2-L) and several isoforms including a short splice variant lacking the Ca²⁺-permeable pore region (TRPM2-S), which functions as a dominant negative. Treatment with H₂O₂, a model of oxidative stress, or TNF results in reduced cell viability. Expression of TRPM2-L and TRPM2-S was modulated by retroviral infection. U937-ecoR cells expressing increased levels of TRPM2-L were treated with H₂O₂ or TNF, and these cells exhibited significantly increased intracellular calcium ([Ca²⁺]_i), decreased viability, and increased apoptosis. A dramatic increase in cleavage of caspases 8,9,3,7, and PARP was observed, demonstrating a downstream mechanism through which cell death is mediated. Bcl-2 levels were unchanged. Inhibition of the [Ca²⁺]_i rise with the intracellular Ca²⁺ chelator BAPTA blocked caspase/PARP cleavage and cell death induced following activation of TRPM2-L, demonstrating the critical role of [Ca²⁺]_i in mediating these effects. Down regulation of endogenous TRPM2 by RNA interference or increased expression of TRPM2-S inhibited the rise in [Ca²⁺]_i, enhanced cell viability, and reduced numbers of apoptotic cells after exposure to oxidative stress or TNF, demonstrating the physiologic importance of TRPM2. Our data show that one mechanism through which oxidative stress or TNF mediate cell death is by activation of TRPM2, resulting in increased [Ca²⁺]_i, followed by caspase activation and PARP cleavage. Inhibition of TRPM2-L function by reduction in TRPM2 levels, interaction with TRPM2-S, or calcium chelation antagonizes this important cell death pathway.