The guideline group was selected to be representative of UK-based medical experts with an interest in myeloproliferative neoplasms and eosinophilia. PubMed and EMBASE were searched systematically for publications in English until August 2015 using the key words eosinophilia, hypereosinophilia, eosinophilic leukaemia and HES. The writing group produced the draft guideline, which was subsequently revised by consensus by members of the General Haematology and Haemato-oncology Task Forces of the British Committee for Standards in Haematology (BCSH). The guideline was then reviewed by a sounding board of UK haematologists and representatives from the Nordic MPN Study Group, the BCSH and the British Society for Haematology (BSH) Committee, and comments were incorporated where appropriate. The ‘GRADE’ system was used to quote levels and grades of evidence, details of which can be found in Table 1. The objective of this guideline is to provide healthcare professionals with clear guidance on the investigation and management of eosinophilia.