A 63-year-old man with smoldering adult T-cell leukemia (ATL) which became acute was admitted. During chemotherapy, he experienced epigastric pain and fever due to neutropenia. The combination therapy of antimicrobials and rhG-CSF was ineffective and he died. Autopsy revealed systemic invasion of ATL cells. The stomach findings resembled those of phlegmonous gastritis, a rare form of bacterial gastritis, along with diffuse, mucosal necrosis with hemorrhage. The pathogenesis of necrotizing gastritis remains to be elucidated. The patient had also received histamine H2 antagonist for gastric ulceration, which might have influenced the gastric bacterial flora.