A 53-year-old man was hospitalized for the development of watery diarrhea associated with decreased appetite and progressive weight loss and was found to have leukocytosis. The white blood cell count was 14,600/microliters with 66 percent abnormal lymphocytes. Serum anti-HTLV-I was positive and monoclonal insertion of HTLV-I provirus into the atypical cell genome was confirmed with the southern blotting hybridization technique. A diagnosis of ATL was made. Examination of fresh stool specimens revealed Isospora belli (I. belli) oocysts. Initial treatment for I. belli consisted of oral trimethoprim 160mg and sulfamethoxazole 400mg given twice daily for nine days. Diarrhea ceased within 2 days of the start of treatment, but I. belli oocysts were again detected after 20 days. Trimethoprim 160mg and sulfamethoxazole 400mg were reinstituted four times daily for 10 days, and then twice daily for 21 days. The clinical response was again dramatic, with rapid clearance of oocysts from the stool. There has been no recurrence of diarrhea. The patient's leukemia was refractory to chemotherapy; the white blood cell count continued to rise, pneumonia developed, and the patient died. I. belli is a previously unrecognized opportunistic pathogen that must be considered in the clinical setting of chronic diarrhea in patients with ATL.