Eosinophilia is a common problem in travellers and migrants returning from the tropics. It usually signifies an underlying helminth infection, although it is still important to bear in mind the possibility of a non-infectious cause in this group of patients. It is frequently asymptomatic, but deserves investigation to prevent significant disease in the future. The presentation of helminth infections can differ between individuals who have migrated from endemic areas – who can have heavy infections with a subacute course – and travellers, in whom acute presentations are usual. Negative tests should be repeated if they were performed within 3 months of return from the tropics. Here, we consider the most common and potentially serious causes of eosinophilia in returned travellers. The diagnosis of eosinophilia can be incidental, so we first consider when and how to investigate asymptomatic eosinophilia. We then consider the symptomatic presentation of eosinophilia, organized in syndromes that the physician is likely to encounter (fever and respiratory, gastrointestinal, genitourinary, neurological and dermatological presentations).