A 75-year-old man visited our hospital complaining of heartburn in November 1997. Gastroscopical examination revealed ulcerous protruding extended from the gastric antrum to body and a flat, elevated lesion in the greater curvature of the stomach. Biopsy specimens revealed a CD4-positive malignant lymphoma. The serum anti-human T-lymphotrophic virus type I (HTLV-I) antibody test was positive. He was diagnosed as having primary gastric adult T-cell leukemia/lymphoma (ATLL; acute type). Complete remission was achieved with chemotherapy. In December 1998, the patient experienced a relapse. The lesions were limited to the region between the upper gastric body and the fornix and disappeared with radiation therapy. A second relapse was detected in the gastric greater curvature and descending portion of the duodenum in May 1999 but spontaneously disappeared in 5 months. The third relapse in May 2000 was systemic. Monoclonal integration of the HTLV-I provirus was observed in DNA extracted from ascitic lymphocytes. Chemotherapy was resumed, but the response was poor. The patient subsequently died of respiratory failure as a result of pneumonia. Although gastrointestinal involvement is frequent in ATLL, this appears to be a rare case of an idolent clinical course with lesions limited to the stomach and duodenum for 30 months.