Activation of ST2, an orphan member of the IL-1 receptor family for 16 years, drives T helper type 2 (T(H)2) responses. The cytokine IL-33 is the specific ligand for ST2. IL-33 recapitulates much of the existing data that ST2 promotes T(H)2-type responses. The caspase-1 processing of precursor IL-33 provides a therapeutic target to control allergic diseases.