Adult T-cell leukemia/lymphoma (ATLL) is a malignancy of mature T-cells caused by the human T-cell lymphotrophic virus type I (HTLV-D. HTLV-I is endemic in some areas in Japan, the Caribbean basin, and Africa but has low prevalence in South Korea. Patients with ATLL are susceptible to opportunistic infections such as cytomegalovirus (CMV) infection, but CMV infection in chronic ATLL is uncommon. Reported herein is a case involving a 44-year-old woman with chronic ATLL who presented the symptoms of fever and diarrhea. She was suspected to have acute-type ATLL but was later diagnosed with CMV colitis.