A 79-year-old male patient underwent esophagogastroduodenoscopy, which revealed a reddish lesion, 10mm in diameter, presenting as a surface recess in the angular incisure. He was diagnosed with gastric follicular lymphoma. Positron emission tomography-computed tomography revealed metastasis to the mediastinal lymph node, although the tumor size was small. Hence, we did not administer any treatment and continued following up. After 8 months, multiple enlarged lymphoma lesions in the stomach and a mass with ulceration on the oral side of the duodenal papilla were observed. The tumor had transformed into diffuse large B-cell lymphoma; therefore, chemotherapy was initiated. The patient has remained recurrence-free for 55 months after treatment.