When the number of eosinophil granulocytes in blood increases, the cause is not always easy to disentangle. This review highlights the symptoms of rare clonal and common reactive diagnoses, how to approach the patient clinically, and how to implement the armamentarium of available tests in order to identify the correct diagnosis and offer the proper treatment. Two referral centres for eosinophilia have been established in Denmark to support this activity by a collaboration between all departments of haematology and the relevant specialities, meeting the manifestations of eosinophilia.