[en] Iodine-131 therapy is one of the most common treatments for hyperthyroidism and thyroid cancer. Data on man exposed to 131-I are still scarce, and there is concern as to its possible genetic and carcinogenic effects. No overall increased cancer risk has been observed in patients receiving 131-I therapy for hyperthyroidism. Two studies have reported increased risks for leukemia after 131-I therapy for throid cancer. Different sites have been demonstrated to be at increased cancer risk in different record-linkage studies of thyroid cancer patients, e.g. bone-marrow, breast and kidney, nervous tissue, and connective tissue. However, the findings are not consistent. This article presents results from a Swedish multicenter cohort study analyzing risks of second primary tumors in patients treated with 131-I for hyperthyroidism or thyroid cancer. 10 refs