Inclusion criteria:

• ≥ 12 years of age

• Meets the diagnostic criteria for Hyper Eosinophilic Syndrome as defined by:

  1. Eosinophilia >1500/ul 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).

  2. Subjects with eosinophilia of >1500/ul for less than 6 months who meet the other criteria for HES accompanied by clear evidence of eosinophil tissue infiltration and with exclusion for secondary causes of eosinophilia as above.

     3.Has ongoing HES, as judged by documented treatment failure defined as life-threatening or severely disabling signs of HES in association with documented eosinophil count ≥ 1000µl on > 1 occasion with documented failure to respond to at least one agent in each category of available therapies; a) systemic corticosteroid, b) cytotoxic agent, c) alpha interferon, and d) Imatinib for FIP1L1-PDGRF-alpha fusion gene positive cases, at the appropriate duration and dose.

     OR, MEETS THE FOLLOWING CRITERIA

• Has previously received mepolizumab through compassionate access, investigator initiated clinical trial or who previously received the Schering-Plough anti IL-5 (SCH55700) compound and who has shown evidence of clinical.

Exclusion Criteria:

1. Those who have withdrawn from the MHE100185 Phase III study due to treatment failure and who are eligible to receive open label mepolizumab through the MHE100901 extension study in order to maximize the efficacy and safety data collected through the clinical trial.
2. Female subjects not using an effective method of contraception Consistent and correct use of one of the following acceptable methods of birth control for one month prior to the start of the investigation product and three months after the last dose: Male partner who is sterile prior to the female subject's entry into the study and is the sole sexual partner for the female subjects; Implants of levonorgestrel; Injectable progestogen; any intrauterine device (IUD) with a documented failure rate of less than 1% per year; oral contraceptives (either combined or progestogen only); Double-barrier methods including diaphragm or condom with a spermicide.
3. Pregnant or lactating females (but see concept of compassionate use in life-threatening disease below)
4. Subjects with eosinophilic leukaemia or pre-leukemia in whom anti-IL-5 is not expected to be effective and in whom exacerbation may occur on cessation of anti-IL-5.
5. Subjects with severe/life-threatening underlying disease unrelated to HES (i.e. organ failure, life expectancy less than 3 months), however, see concept of compassionate use in life-threatening disease below).
6. Subjects with known lymphoma, hematological malignancy advanced and/or metastatic tumors, subjects receiving chemotherapy, radiotherapy and IL-2 due to potential risk of driving tumor growth with use or withdrawal of mepolizumab.
7. Subjects with history of serious adverse event or allergic reaction to anti-IL5 or other antibody therapy.
8. Subjects with current drug or alcohol abuse where uncertain compliance with medication causes safety risk.
9. Subjects currently receiving chemotherapy, radiotherapy or other investigational interventions.
10. Subjects with other conditions associated with eosinophilic pathological process such as Churg-Strauss, Wegener's Granulomatosis, atopic disorders, hypersensitivity reactions to parasitic infections, eosinophilic gastroenteropathies, will not be eligible for this compassionate use program which is restricted to HES.