Churg-Strauss syndrome is a rare diffuse vasculitis that is almost invariably accompanied by severe asthma. Although overall prognosis is good, and treatment with prednisone alone or in combination with immunosuppressive drugs is usually successful, severe asthma typically persists. Diffuse organ involvement of Churg-Strauss syndrome, especially cardiovascular and rare involvement of the CNS and renal system, suggests a poorer prognosis than usual, and can be fatal. The cause of Churg-Strauss syndrome is unknown, but its characteristic histological findings and association with asthma distinguish it from other vasculitides. Controversy surrounds the use of asthma drugs-especially antileukotrienes--and development of the disorder. We review the epidemiological evidence for an association of drug treatment with Churg-Strauss syndrome, the diverse diagnostic and pathological criteria for this syndrome, and treatment options.