A 47-year-old man had diarrhea in 1965. Four years later, malabsorption syndrome was diagnosed and the patient was found to have mild lymphocytosis. Abdominal lymphoma was suspected, but exploratory laparotomy was normal except for partial villous atrophy of small intestine and slightly enlarged mesenteric lymphnodes which were normal microscopically. In vitro lymphocyte blastformation with phytohemagglutinin was depressed markedly throughout the course and the result predicted the developement of malignancy of the lymphocytic system. Infection of Isospora betli was found thereafter, and sul-famethoxazole was quite effective for diarrhea. In August, 1974, he noticed cervical lymphadenopathy for the first time and it was diagnosed as undifferentiated type of malignant lymphoma. He died in December, 1974. In this case diarrhea was most probably caused by the intestinal infection of Isospora belli without obvious lymphoma. The symptom was swept away by peroral sulfamethoxazole. In this patient coccidiosis was presumably induced and prolonged by suppression of cellular immunity which might have already begun to progress at the onset of diarrhea.