Churg-Strauss syndrome was originally called "allergic granulomatosis and angiitis," describing the combination of eosinophilic inflammation, extravascular granulomas, and necrotizing vasculitis occurring in patients with severe asthma. It is now classified as a small-vessel vasculitis and, together with Wegener's granulomatosis and microscopic polyangiitis, as one of the vasculitides associated with antineutrophil cytoplasmic autoantibodies (ANCA). Glucocorticoid-sparing agents used in the treatment of asthma, such as leukotriene receptor antagonists, may unmask this particular form of vasculitis as oral glucocorticoids are withdrawn. ANCA occur in 40-75% of patients with active disease and typically react with myeloperoxidase. Patients' symptoms are defined by various degrees of eosinophilic inflammation and necrotizing vasculitis, which may affect any organ. On presentation, Churg-Strauss syndrome needs to be differentiated from other eosinophilic pneumonias, idiopathic hypereosinophilic syndrome, and Wegener's granulomatosis and microscopic polyangiitis. Churg-Strauss syndrome remains a rare disease with a poorly understood pathogenesis. Treatment consists primarily of glucocorticoids. Patients who have ANCA at the time of presentation should be treated according to the treatment principles for ANCA-associated vasculitides. However, the exact role of glucocorticoid-sparing immunosuppressive agents and treatment options for refractory disease remain poorly studied.