Sixteen patients with Churg-Strauss syndrome (CSS), a disorder characterized by hypereosinophilia and systemic vasculities which complicate preexisting asthma, were analyzed. The mean duration of asthma before CSS was 8 years; peripheral blood eosinophilia was always greater than 1,900/microliters and exceeded 5,000/microliters in 14 cases. The clinical manifestations were the following: 16 in generally poor condition with fever; 12 peripheral neuropathies; 11 cutaneous lesions; 9 pericardial or myocardial involvement; 9 digestive disorders; 9 muscular or articular diseases; 5 renal involvement, all associated with the vasculitis; 7 upper respiratory tract disorders. Chest radiographs showed pleuropulmonary or cardiac anomalies in 11 patients. The diagnosis was confirmed histologically in 11 cases, however, no clinical, biological or evolutive differences were observed between these patients and those with negative biopsies (5). Follow-up for 6.35 +/- 5.55 years was characterized by relapses always preceded by increased eosinophilia. Fourteen patients were successfully treated with corticosteroids, associated with cyclophosphamide in 7 of them. Five-year survival was 87%. Four deaths occurred, all CSS-associated, two because of a poorly adapted therapeutic regimen. The need for rapid and effective treatment must be stressed. The diagnosis can be made based on clinical manifestations alone before histological confirmation can be obtained.