Anti-Interleukin-5 (IL-5) Study for Hypereosinophilic Syndrome

Inclusion Criteria: * Adequate renal, cardiac, and hepatic function * Not pregnant or breastfeeding * A diagnosis of hypereosinophilic syndrome such as: * Idiopathic hypereosinophilic syndrome; * Eosinophilia myalgia syndrome; * Eosinophilic gastroenteritis; * Churg-Strauss syndrome; * Eosinophilic cellulitis; * Benign hypereosinophilia; or * Eosinophilic esophagitis. * Maintained on, or in need of, the following: glucocorticoids, interferon, methotrexate, hydroxyurea, cytoxan, 6-mercaptopurine, or vincristine * Blood eosinophil counts greater than 750 cells/mcl Exclusion Criteria: * Creatinine \> 3 X upper limit for age * AST \> 5 X upper limit for age * Platelet count \< 50,000/mm3 * Cardiac function: 1. NYHA class IIIb or IV (patients with shortness of breath or fatigue with mild exertion or at rest); 2. Patients with symptomatic supraventricular or ventricular arrythmias requiring treatment; 3. Patients requiring IV heart failure medications; 4. Angina or acute myocardial infarction * History of renal, hepatic, endocrine, oncologic, cardiovascular disease, or any other condition which, in the opinion of the Investigator, would jeopardize the safety of the patient or impact on the validity of the study results * History of allergic or adverse response to previous antibody type therapy * History of allergic or adverse response to anti-IL-5 therapy * Evidence of, or history of, a parasitic infection (within past 1 year) * Participation in a previous clinical trial involving an investigational agent within 30 days prior to study initiation * Receipt of anti-IL-5 therapy in the past * Blood donation of 450 ml (1 pint) or more within 30 days prior to study initiation * Abnormal or unusual diet or substantial changes in eating habits within 30 days prior to, or during, the study. * Patients who do not have eosinophil levels \> 750 cells/mcl after the run-in period. Patients who have only eosinophilic infiltration of tissue are eligible if they do not develop an eosinophil level \> 750 cells/mcl, assuming that they meet all other eligibility requirements. * Positive serum pregnancy test * Breastfeeding * Unable to use effective birth control methods for duration of study * Unable or unwilling to give voluntary informed consent/assent