Churg-Strauss syndrome (CSS) is a disease characterised by hypereosinophilia and systemic vasculitis occurring in patients with asthma and allergic rhinitis. In the course of CSS three phases may be distinguished. The prodromal phase, which may persist for many years, consists of allergic disease. The second phase is characterised by peripheral blood eosinophilia and eosinophilic tissue infiltrates that produce a clinical picture diagnosed as Loeffler's syndrome, chronic eosinophilic pneumonia or eosinophilic gastroenteritis. The third phase is dominated by systemic vasculitis in which skin, cardiovascular system, gastrointestinal tract and peripheral nervous system are frequently involved. Renal disease in CSS is less common and generally less severe than that classical polyarteritis nodosa and Wegener's granulomatosis. Genitourinary tract may be involved, too. In the postvaculitic phase, allergic rhinitis and asthma usually persist and clinical picture is characterised by the consequences of the vasculitic illness, most commonly in form of neuropathy and hypertension. The pathogenesis of CSS is unknown but its association with asthma and allergic rhinitis may indicate an abnormal immune reactivity. The recently reported association with antineutrophil cytoplasmic antibodies with antimyeloperoxidase specificity may suggest a their role in the pathogenesis of the disease. An important role may be played by eosinophils, too. The main therapy is that with corticosteroids, possibly in association with immunosuppressive drugs.