A retrospective study was carried out on 119 Caucasians who were admitted to the Hospital for Tropical Diseases, London, for investigation of eosinophilia on return from the tropics. In 46 patients (38.7%) a definite parasitological diagnosis was made, and in all those followed up eosinophilia decreased with appropriate chemotherapy. In those patients where no cause for eosinophilia could be found, results suggest that empirical treatment with a broad spectrum anthelmintic agent may be justified in an attempt to produce a normal blood picture.