Little is known about the origin of vasculitis in CSS or about the cause of tissue eosinophilia. Recently, CSS has been described as “vasculitides strongly associated with atopic disorder,” or type I reactions in the classification of Gell and Coombs. Activated TH2 lymphocytes play a central role through their production of cytokines, such as interleukins 4, 5, and 13, mediating the accumulation of mast cells, basophils, and, especially, eosinophils. Activation of eosinophils is a central feature of CSS, whereas eosinophilic cationic protein, eosinophil-derived neurotoxin, and lipid mediators seem to play a major role in the induction of eosinophilic vasculitis. In our study, increased concentrations of IgE were observed in all patients, and these levels showed a propensity to normalize during periods of disease remission. Our patients with CSS responded very well to steroid treatment, and the response was often dramatic. These findings, together with eosinophil counts and IgE measurements, may be useful in following disease activity and response to therapy. The findings of the present study, when considered in conjunction with those published by others, suggest that factors, including eosinophil counts and IgE levels, have a pathogenic central role in CSS.