Churg-Strauss syndrome (CSS) is a form of primary vasculitis characterized by allergy and angiitis. In the organ systems involved (lung, heart, peripheral nervous system, and so forth), eosinophilic infiltration can be found. Eosinophilia and normochromic normocytic anemia are leading laboratory findings together with elevated IgE. New seromarkers for the activation of endothelial cells, lymphocytes, and eosinophils (soluble thrombomodulin, soluble interleukin-2 receptor, eosinophil cationic protein) may be able to predict a relapse. Antineutrophil cytoplasmic antibodies are found in only approximately 50% of all patients with CSS, and their diagnostic value is questionable. Etiologically, hyperresponsiveness to an antigenic stimulus seems to underlie the syndrome. In asthmatics, cysteinyl leukotriene receptor type 1 antagonists are reported to trigger the disease. Cytokine profile findings on the cells involved in CSS remain contradictory. Some think CSS may be a Th2-mediated disease; its pathophysiology is not known fully. Interleukin-5 and tumor necrosis factor-&agr; are elevated in serum and fluid of bronchoalveolar lavage, suggesting target cytokines for future treatment protocols. Treatment consists of glucocorticoid monotherapy. Data on outcome and effectiveness is lacking for other immunosuppressive regimens, such as cyclophosphamide or glucocorticoid plus cyclophosphamide. Treatment with interferon-&agr; has been effective in patients refractory to glucocorticoid plus cyclophosphamide.