Inclusion Criteria:

• In accordance with local procedures, written informed consent/assent can be obtained from the patient or legally authorized representative.
• ≥ 12 years of age at the time of signing the informed consent/assent.
• Meets the diagnostic criteria for HES as defined by:
  • Eosinophilia >1500 cells/μl for at least 6 months with evidence of symptoms and signs of organ system involvement or dysfunction that can be directly related to eosinophilia (with no evidence of parasitic, allergic or other recognised causes of eosinophilia such as connective tissues disease, malignancy); or
  • Eosinophilia of >1500 cells/μl for less than 6 months and meet the other criteria for HES accompanied by clear evidence of eosinophil tissue infiltration and with exclusion of secondary causes of eosinophilia as above.
• Patients meeting all three of the following criteria will be eligible:
  • The indication, HES, is a seriously debilitating or life-threatening disease;
  • There is no satisfactory alternative treatment: documented failure (lack of efficacy or a contra-indication) to at least 3 standard therapies (corticosteroids, cytotoxic agents, immunomodulatory therapy, and Imatinib mesylate) at the appropriate duration and dose or demonstrated clinical benefit from prior treatment with mepolizumab; and
  • There is reason to believe that the benefit:risk ratio for mepolizumab in the indication is positive.

Exclusion Criteria:

• Patients without HES but with other conditions associated with eosinophilic pathological processes such as eosinophilic granulomatosis with polyangiitis [EGPA], Wegener’s granulomatosis, atopic disorders, parasitic infections, eosinophilic
• gastroenteropathies. 
• Female patients of childbearing potential who are not using a highly effective
• method of contraception:
  • Consistent and correct use of an acceptable method of birth control for one month prior to the start of the investigational medicine and until 16 weeks after the last dose.
• Pregnant or lactating females
• Patients with severe/life-threatening underlying disease unrelated to HES where life expectancy is estimated to be less than 3 months.
• Patients with a history of or current malignancy:
  • Patients with a history of or current lymphoma;
  • Patients with current malignancy or previous history of cancer in remission for less than 12 months prior to the first dose. Patients that had localized carcinoma (i.e., basal or squamous cell) of the skin which was resected for cure will not be excluded.
• Patients with history of serious allergic reaction (hypersensitivity/anaphylaxis) to anti-IL5 or other antibody therapy or known or suspected hypersensitivity to any component of mepolizumab, leading to treatment discontinuation.
• Patients with current drug or alcohol abuse where uncertain compliance with the protocol and/or with the medical management instruction of the treating physician may cause safety risk.
• Patients who have received treatment with an investigational agent (biologic or nonbiologic, excluding mepolizumab) within the past 30 days or 5 drug half-lives whichever is longer, prior to the administration of mepolizumab under this protocol.  The term “investigational” applies to any drug not approved for sale in the country in which it is being used or investigational formulations of marketed products.