5-[124I]Iodo-2'-deoxyuridine ([124I]IUdR) was routinely produced by direct electrophilic labelling of 2'-deoxyuridine with 124I of high specific activity (12 Ci/micromol) in an Iodogen-coated ReactiVial, followed by purification on a Sep-Pak C-18 cartridge. The radiochemical purity was determined by TLC on a Silicagel-60 plate and by reverse-phase HPLC on a RP-18 column. Based upon 45 syntheses, the yield ranged from 45% to 65%. The radiochemical impurity of [124I]IUdR was determined at 2.9% by TLC (mainly iodate) and 4.3% by HPLC. The chemical stability of the solvated formulation allowed a time window of 2 days following end of synthesis (EOS) for chemical application, based upon the required 95% radiochemical purity grade of [124I]IUdR. The labelled compound was routinely used for the clinical determination of cell proliferation in glioma patients by positron emission tomography.