131I is the accepted thyroid scanning agent in spite of the substantial radiation dose administered to the gland. Other isotopes of iodine (1,2) have been suggested but none have proved ideal. When Harper (3) introduced technetium 99m it was shown that satisfactory thyroid scans were possible with a marked reduction in the radiation dose (4–6). Because of this advantage we have compared the two isotopes for routine use as thyroid scanning agents. Method We evaluated the usefulness of technetium by performing thyroid scans with both 99mTcO4 and 131I on ninety-eight patients. First we made a 99mTc scan one hour after the intravenous administration of one to two millicuries of 99mTc. Then we administered orally fifty microcuries of 131I and made a scan with this nuclide the next day. At least a twenty-four-hour I31I uptake was measured. Both scans were performed with a scanner3 that had a crystal 5.1 cm thick and 7.6 cm in diameter. A 721-hole focused collimator, especially designed for use with low energy-e...