We describe three cases of adult T-cell leukemia/lymphoma (ATL) with malignant cell infiltration into the stomach. Case 1, a 63-year-old man, had a large ulcerative lesion on the lesser curvature of the upper stomach. Case 2, a 44-year-old man, had an irregular depressed lesion simulating a IIc-type early gastric cancer on the posterior wall of the mid portion of the stomach. Case 3, a 58-year-old man, had multiple irregular depressed lesions on the lesser curvature of the lower part of the stomach. In all cases, histologic examinations of the biopsy specimens confirmed ATL-cell infiltration into the stomach. In cases 1 and 2, repeated roentgenographic and endoscopic examinations revealed the characteristic changes of the gastric lesions over the course of time.