The idiopathic hypereosinophilic syndrome (HES) is an infrequent entity defined by an eosinophil count > 1.5 x 10(9)/l and the specific infiltration of different organs not attributable to another disease. Its prognosis is marked by the complications derived from visceral, particularly cardiac, infiltration determining a median survival of less than one year if treatment is not administered. Although a notable prolongation in survival may be achieved in these patients with the administration of glucocorticoids and cytolytic drugs, mainly hydroxiurea, there remains a group of patients who do not respond to these therapeutic measures, thus leading to alpha interferon trials with promising results. A favourable experience with the use of alpha interferon in two patients with IHS resistant to conventional treatment is reported. The first patient presented a picture characterized by diarrhoea, palpitations and effort dyspnea demonstrating heart involvement suggestive of eosinophilic infiltration on echocardiographic study. The second patient presented dyspneic night cough and generalized pruritus in addition to anemia and thrombocytopenia. The administration of alpha interferon (3 x 10(6) U/3 days a week and 3 x 10(6) U/day, respectively) was followed in both cases by the rapid disappearance of the symptomatology, normalization of the hematologic parameters and a reduction of the eosinophilia to values of less than 1 x 10(9)/l. This response has maintained over the two years of control in which the patients have received alpha interferon as a maintenance treatment.