Leukemia has rarely been reported as a late complication of 131I therapy, occurring mostly after cumulative doses of 800 mCi. We observed two cases of acute myeloid leukemia (AML) after 131I therapy for hyperthyroidism and thyroid carcinoma, respectively. The first patient was a 45-year-old woman treated with a single dose of 27 mCi 131I for hyperthyroidism. She developed AML (FAB M2) 14 months after receiving 131I; the second patient was a 44-year-old man affected by refractory thyroid carcinoma who received a total dose of 1 Ci 131I plus radiotherapy and developed AML (FAB M6) 8 years after the first exposure to 131I. Although it is a very rare event, the occurrence of leukemia after 131I treatment should be kept in mind, considering the widespread use of 131I, particularly in the treatment of hyperthyroidism, and the unfavorable outcome of secondary leukemia.