Patients with eosinophilia (an increased number of eosinophilic granulocytes > 0.5 × 109/l) are seen in all medical specialties. In practice, it may prove difficult to distinguish between primary eosinophilia (caused by clonal haematological condition) and secondary eosinophilia, which is driven by activation of eosinophils due to autoimmunity, inflammation, infection, malignancy or allergy. In the April issue of Danish Medical Journal, Hougaard et al. describe the diagnostic process and causes of eosinophilia in patients referred for a second opinion to a highly specialized tertiary centre. The authors report that 59% of the patients had secondary eosinophilia, whereas 2% presented with myeloid neoplasm with PDGFRA rearrangement, 31% with idiopathic hypereosinophilic syndrome and 8% had idiopathic hypereosinophilia. They conclude that flow cytometry was the most clinically applicable investigation.