Authors report about a case of Strongyloides stercoralis infestation. The patient, a 65-year old man, presented with a clinical history of eosinophilia but without symptomatology. Several stool specimen showed the presence of rhabditiform larvae of S. stercoralis and after therapy the patient had a normalization of the white blood ceIls count. The authors wish to point out the importance of the diagnosis because a change in immune status may convert a previously asymptomatic infection to hyperinfection.