IL-33 is a chromatin-associated cytokine of the IL-1 family that has recently been linked to many diseases, including asthma, rheumatoid arthritis, atherosclerosis, and cardiovascular diseases. IL-33 signals through the IL-1 receptor-related protein ST2 and drives production of pro-inflammatory and T helper type 2-associated cytokines in mast cells, T helper type 2 lymphocytes, basophils, eosinophils, invariant natural killer T cells, and natural killer cells. It is currently believed that IL-33, like IL-1β and IL-18, requires processing by caspase-1 to a mature form (IL-33_112–270) for biological activity. Contrary to the current belief, we report here that full-length IL-33_1–270 is active and that processing by caspase-1 results in IL-33 inactivation, rather than activation. We show that full-length IL-33_1–270 binds and activates ST2, similarly to IL-33_112–270, and that cleavage by caspase-1 does not occur at the site initially proposed (Ser₁₁₁), but rather after residue Asp₁₇₈ between the fourth and fifth predicted β-strands of the IL-1-like domain. Surprisingly, the caspase-1 cleavage site (DGVD₁₇₈G) is similar to the consensus site of cleavage by caspase-3, and IL-33 is also a substrate for this apoptotic caspase. Interestingly, we found that full-length IL-33, which is constitutively expressed to high levels by endothelial cells in most normal human tissues, can be released in the extracellular space after endothelial cell damage or mechanical injury. We speculate that IL-33 may function, similarly to the prototypical alarmins HMGB1 and IL-1α, as an endogenous danger signal to alert cells of the innate immune system of tissue damage during trauma or infection.