Isospora belli infection is frequent in patients with acquired immunodeficiency syndrome in tropical areas. It has also been reported in other immunodepressive diseases, such as lymphoblastic leukemia, adult T-cell leukemia, and Hodgkin's disease. To date, no case of non-Hodgkin's lymphoma-related isosporiasis has been reported in a non-HIV-infected patient. We describe a case of non-Hodgkin's lymphoma with chronic diarrhea due to I. belli. In Europe, I. belli can cause severe chronic diarrhea in patients with malignancies whose country of origin is in an endemic area. Trimethoprim-sulfamethoxazole can provide rapid and prolonged clinical and parasitologic cure.