The fastest and most reliable preparation of radiolabeled 5-iodo-2'-deoxyuridine ([123/125/131I]IUdR) is accomplished by iododemetallation. We describe a series of improvements to a previously published synthesis of radiolabeled IUdR by demercuration whereby [123/125/131I]IUdR is synthesized instantly following the incubation of an aqueous solution of the chloromercury precursor with Na123/125/131I in the presence of Iodogen. The total procedure is completed in 5 min and does not require purification. Mercury content is low and HPLC indicates the total transformation of iodide into IUdR with no detectable UV-absorbing by-products.