Eosinophilia was a frequently detected incidental finding during a prospective study of malaria seroepidemiology in Thailand. Blood eosinophil counts were performed every 3 months for a year in 823 Thai soldiers on border guard duty in a malaria endemic area. Soldiers developing malaria were admitted to hospital and more frequent eosinophil counts were done. P. falciparum parasitemia suppressed preexisting eosinophilia but eosinophilia returned following treatment. P. vivax and mixed infections had a similar but less marked effect on the peripheral blood eosinophil count. Eosinophilia in persons from a malaria endemic area may represent a normal late response to malaria infection.