Gastric mucosal lesions in 13 patients with adult T-cell leukemia (ATL) were endoscopically studied at least at 2 occasions during chemotherapy for these patients. X-ray examinations by using barium meal showed deformities in 10 patients and abnormal mucosal pattern in 11 out of 13 patients with ATL. The endoscopic examinations revealed gastric ulcers in 5 patients, erosion in 8, diffuse bleeding in 2, tumorous mucosal folds in one, and submucosal tumor in one patient. In 9 out of 12 ATL patients, pathological study revealed that ATL cells had invaded the mucosal area of the stomach. The ulcers and erosions were improved by chemotherapy in 3 out of 5 and 5 out of 8 patients, respectively, whereas the erosion was not reduced in 2 and even aggravated in one patient. Interestingly, the gastric lesions formed by ATL cell invasion were improved by the administration of anticancer agents in 5 out of 9 patients. However, these lesions reappeared together with systemic lesions of ATL.