RNASE3 and idiopathic hypereosinophilic syndrome: In this pilot prospective single-centre cohort study, we analysed clinical (age, years of disease, steroid therapy) and laboratory (absolute eosinophil count, total IgE antibodies, IgE antibodies against Staphylococcus aureus enterotoxins, serum eosinophil cationic protein, serum immunoglobulin free light chains k and λ and their ratio) data obtained from 21 patients suffering from idiopathic hypereosinophilic syndrome from June 2023 to December 2024.