Ionizing radiation including I(131) might produce chromosomal translocation, causing hematologic malignancy. The incidence of leukemia following radioactive iodine treatment for thyroid cancer has been reported to be approximately 0.1 to 2.0% in Western countries, whereas fewer cases have been reported in Korea. We hereby report four cases of secondary hematologic malignancy, who received iodine therapy for thyroid cancer after thyroidectomy: two cases of acute lymphoblastic leukemia with t(9;22)(q34;q11.2), a case of MDS with 5q deletion, and a case of MDS with normal karyotype. Three cases of hematologic malignancy have developed after cumulative dosage of less than 800 mCi. The treatment intervals in two cases were less than 12 months, and the other two cases had I(131) therapy only once. Assessment of causality using the Naranjo probability scale for adverse drug reactions showed that a 'possible' relationship existed between the use of I(131) and secondary hematologic malignancy in all of the four cases in this report.