Patients with hypereosinophilic syndrome show persistent eosinophilia without recognised cause. We treated five such patients with 100 mg imatinib mesilate (formerly STI-571) daily; four male patients with normal serum interleukin 5 showed complete haematological responses; a female patient who did not respond had raised serum interleukin-5 concentrations. One patient developed leucopenia after 4 days of treatment; counts returned to normal when treatment was stopped. After 1 month, eosinophilia recurred; with further treatment for 2 days, eosinophil counts again became normal. All patients who responded stopped other treatments and reduced imatinib mesilate to 200 mg per week. This drug effectively controls eosinophilia in patients with hypereosinophilic syndrome and normal interleukin-5 concentrations.