3'-deoxy-3'-[F-18]fluorothymidine ([F-18]FLT), a cell proliferation positron emission tomography (PET) tracer, has been shown in numerous tumors to be more specific than 2-deoxy-2-[F-18]fluoro-d-glucose ([F-18]FDG) but less sensitive. We studied the capacity of a nontoxic concentration of 5-fluoro-2'-deoxyuridine (FdUrd), a thymidine synthesis inhibitor, to increase uptake of [F-18]FLT in tumor xenografts.