The recently recognized cytokine interleukin-33 and its receptor ST2 play a favorable role during atherogenesis by inducing a Th1 --> Th2 shift of the immune response. IL-33 also protects the failing human heart from harmful biomechanical forces which lead to cardiomyocyte hypertrophy and exaggerated interstitial fibrosis. IL-33 inevitably displays side effects common to other Th2 cytokines, the most grave of which is a predisposition to allergic reactions. IL-33 is a nuclear transcription factor of endothelial cells. As such, it is abundant in nonproliferating vessels. Its down-regulation is required for angiogenesis, which may be profitable in wound healing or deleterious in tumor growth.