The patients were treated with 9 μg IFN-αcon (equivalent to 3 million units of IFN-α2b) administered subcutaneously thrice weekly. In all patients, IFN-α induced a significant clinical improvement with respect to clinically relevant parameters of the disease (asthma, rhinitis, sinusitis, hypereosinophilia) with partial (patients BG and UG) and complete remissions (patients KV and BK). In patient KV, both the symptoms of the hemorrhagic cystitis and the Cushing syndrome (weight reduction of 19 kg) disappeared. Also, the peripheral polyneuropathia in patient KV continues to improve since initiation of therapy. The above improvements were achieved despite tapering of oral steroids in all patients. In two patients (patients KV and BK), corticoid treatment could be discontinued. Side effects occurring 2 to 12 h after injection included arthralgia, nausea, malaise, and fever, but were transient and decreased within 3 to 4 weeks. One patient (patient UG) acquired a hyperthyroidism 5 months after initiation of IFN therapy.