Radioactive iodine ( 131 I) has been used in the treatment of thyroid cancer to eliminate residual thyroid tissue after thyroidectomy and to treat metastatic disease. Leukemia has rarely been reported as a late complication of 131 I therapy, occurring most frequently after receiving a cumulative activity of 800 mCi of the radioisotope. Although FAB M3 type (acute promyelocytic leukemia, APL) as the secondary acute leukemia had been rarely reported, recently there have been a few cases of

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... -related APL leukemia seen. We hereby report a case of secondary acute promyelocytic leukemia occurring in a patient who received radioactive iodine therapy for papillary thyroid carcinoma. An assessment of causality using the Naranjo probability scale showed that a possible relationship existed between APL seen in the patient and the use of the radioactive iodine. Although this is a very rare event, strict hematologic follow-up of patients treated with radioactive iodine may be warranted, along with a high index of suspicion in those with coagulopathy. (Korean J Hematol 2006;41:302-305.)