Adult T-cell leukemia/lymphoma of the tongue. A case of posterior tongue lymphoma associated with adult T-cell leukemia (ATL) that occurred as a lesion in the lingual dorsal portion is reported in a 64-year-old woman. Initially, a diagnosis of Hodgkin's lymphoma was considered as no findings associated with ATL except lymphadenopathy and serum anti-ATLA antibodies were present. Combined radiotherapy and chemotherapy were administered with favorable results; however, 4 months later, Pneumocystis carinii pneumonia developed, and 2 months later, generalized lymphadenopathy and hypercalcemia evolved. At this time, a diagnosis of ATL was made. The patient died of renal dysfunction 6 months after the initial presentation. In suspected cases of ATL and malignant diseases of T-cell lineage, namely, malignant lymphoma and mycosis fungoides, the presence of HTLV-1 infection should be confirmed by testing for anti-ATLA antibodies. 