[Show abstract][Hide abstract]ABSTRACT: The idiopathic hypereosinophilic syndrome (HES) comprises a heterogenous group of disorders characterized by marked blood eosinophilia with eosinophilia-associated organ damage. Eight patients with a median age at diagnosis of 42 years (range 19-67) received imatinib mesylate (IM) for FIP1L1-PDGFRα-negative HES resistant to previous conventional treatment. Median number of prior therapies was 3 (range 2-4). Median time from diagnosis to IM initiation was 112 months (range 2-293). Four patients were treated daily with 100 mg IM, whereas the remaining four patients were treated daily with 400 mg IM. Four male patients (50%) achieved complete haematologic response (CHR) after median of 7 days (range 3-150) using 100 mg daily IM (n = 2) and 400 mg (n = 2). Median duration of IM treatment for IM responders was 18 months (range 2-88). No adverse events were reported throughout the treatment duration. Two patients maintained CHR while on 100 mg weekly IM. Four patients (2 men and 2 women) failed IM treatment. Median duration of IM for non-responding patients was 3 weeks (range 3-12). Non-responding HES patients were significantly older and had lower percentage of blood eosinophilia when compared with IM responders. Our results suggest that IM, even at lower than conventional doses, may induce and maintain long-term remission for FIP1L1-PDGFRα-negative HES.