Adjuvant chemotherapy by S-1 following gastrectomy is considered standard treatment in Japan.Analysis of follow-up data have proved the effi cacy of S-1 admin-istration,and that hematological adverse events were relatively rare.Pyrimidine anti-metabolites,including S-1,have shown relatively lower risks for secondary hematological malignancies in comparison to alkylat-ing agents and topoisomerase-Ⅱ inhibitors.We here report a case of therapy-related leukemia after S-1 administration.A patient who had received S-1as the sole adjuvant chemotherapy was diagnosed with acute erythroid leukemia.To the best of our knowledge,our patient represents the fi rst report of S-1 induced acute leukemia.