Churg-Strauss syndrome is a vasculitis associated with asthma and eosinophilia. Respiratory involvement is marked by generally severe and often steroid-dependent late-onset asthma associated with allergic rhinitis and sometimes nasal polyposis and recurrent sinusitis. Asthma generally precedes the systemic vasculitis by a few years. General signs, eosinophilic gastroenteritis, peripheral multiplex neuropathy, cutaneous vasculitis, nephropathy, or arthromyalgia, predominate. Cardiac involvement is often silent but of severe prognosis. The chest X-ray usually shows irregularly delimited and sometimes labile infiltrates. Perinuclear antineutrophil cytoplasmic autoantibodies (ANCA) are found in two-thirds of the patients and strongly suggest the diagnosis. Clinically, the diagnosis is established by the presence of asthma, peripheral eosinophilia > 1.5 G/L, and systemic vasculitis involving at least two extra-pulmonary organs. Histological confirmation is usually necessary (nerve and muscle biopsy), showing small-vessel eosinophilic vasculitis, tissue infiltration with eosinophils, and eosinophilic granulomas. Treatment includes corticosteroids, which should be associated with immunosuppressive agents (cyclophosphamide) in severe cases.