• Safety and tolerability of mepolizumab [ Time Frame: 44 weeks ] [ Designated as safety issue: Yes ]
  

• Demonstrate the steroid sparing effect of mepolizumab [ Time Frame: 44 weeks ] [ Designated as safety issue: No ]
  
• Evaluate overall improvement in CSS via the measures outlined in Study Aims [ Time Frame: 44 weeks ] [ Designated as safety issue: No ]
  

Specific Aims:

1. Document the safety of mepolizumab therapy in patients with CSS.
2. Demonstrate the steroid sparing effect of mepolizumab therapy by decreasing corticosteroid dosage while using this anti-IL5 therapy.

3. Demonstrate the efficacy of anti-IL5 therapy in improving the signs and symptoms of CSS by:

   1. Measuring serum markers of CSS disease activity, including: peripheral eosinophilia, erythrocyte sedimentation rate, anti- neutrophil cytoplasmic antigen, C-reactive protein and IgE levels.
   2. Assessing the activity level of vasculitis via the Birmingham Vasculitis Activity Score
   3. Evaluating asthmatic response via serial peak flow and FEV1 measurements as well as asthma symptom scores using the Juniper scale.
   4. Assessing changes in novel parameters such as fractional excretion of nitric oxide and IL-5 levels.

Inclusion Criteria:

• Age >18 years old
• Diagnosis of Churg Strauss Syndrome
• Maintained on stable corticosteroid dose of at least prednisone 10mg daily (or equivalent) prior to enrollment in study
• If on cyclophosphamide, azathioprine or methotrexate, must be on a stable dose and be able to maintain that dose for the duration of the study

Exclusion Criteria:

• Hypereosinophilic Syndrome
• Wegener's Granulomatosis
• Malignancy
• Parasitic Disease
• Pregnant or nursing
• If female and of child-bearing potential, must have negative pregnancy test prior to each infusion of study medication and must adhere to acceptable method of contraception (with <1% failure rate)
• Any other medical illness that precludes study involvement