The fastest and most reliable preparation of radiolabeled 5-iodo-2'-deoxyuridine ([I-123/125/131]IUdR) is accomplished by iododemetallation. We describe a series of improvements to a previously published synthesis of radiolabeled IUdR by demercuration whereby [I-123/125/131]IUdR is synthesized instantly following the incubation of an aqueous solution of the chloromercury precursor with (NaI)-I-123/125/131 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.