This prospective observational study aimed to assess systemic and skin involvement as well as eosinophilia, anti-neutrophil cytoplasmic antibody (ANCA), and Anti-nuclear antibodies (ANA) serum levels in 20 EGPA patients followed for one year at the University Hospital of Messina, Italy, before starting Mepolizumab, 300 mg. This evidence concerns the gene BTG3 and eosinophilic granulomatosis with polyangiitis.