In the seven years old, male patient with acute myeloblastic leukemia, a sudden diarrhea developed while bone marrow transplantation were planning. The patient's stool samples were examined with respect to pathogen bacteria and fungi and rotavirus. None of them were determined isolated and/or seen. Two stool specimens which were taken in one week interval also were examined with modified trichrome, modified acid fast and modified safranine staining methods. In microscopic examinations, C.cayetanensis oocysts were determined with all three staining methods. After four weeks therapy with trimetoprim-sulfamethoxazole (Bactrim, Roche) (15 mg/kg/day) diarrhea stopped and in the new stool specimens, C.cayetanensis oocysts were not encountered.