The role of the base in the synthesis of 3'-deoxy-3'-[18F]fluorothymidine, [18F]FLT, via nucleophilic substitution of the nosyl group with [18F]fluoride was investigated. The rate of 18F-incorporation into the molecule dramatically changed as a function of the precursor-to-base ratio. In the presence of excess base, the precursor was consumed by elimination before substitution was complete. When the precursor-to-base ratio was optimal, an overall [18F]FLT yield of 30-40% was achieved even if the precursor amount was as small as 8-13 mg.