Positron emission tomography (PET) using the radiotracer 3'-deoxy-3'-[(18)F]fluorothymidine (FLT) can image cellular proliferation in human cancers in vivo. FLT uptake has been shown to correlate with pathology-based proliferation measurements, including the Ki-67 score, in a variety of human cancers. Unlike pathology-based measurements, imaging-based methods, including FLT-PET, are noninvasive, easily repeatable, and less prone to sampling errors. FLT-PET may therefore be a useful tool for assessing tumor aggressiveness, predicting outcome, planning therapy, or monitoring response to treatment. Three recent clinical studies have reported that FLT-PET can accurately predict response very early after the initiation of chemotherapy. Especially with the advent of cytostatic chemotherapy agents, methods of biologically assessing a tumor's response will take on increasing importance, since changes in tumor size will not always be expected. To date, most studies of FLT-PET have focused on validating it as a means of quantifying cellular proliferation and testing its ability to accurately stage cancer. In some settings, FLT-PET has shown greater specificity for cancer than (18)F-fluorodeoxyglucose (FDG)-PET, which can show false-positive uptake in areas of infection or inflammation. However, because of FLT's lower overall uptake and higher background activity in liver and bone marrow, FLT-PET should not be considered a potential replacement for staging by FLT-PET. Instead, FLT-PET should be considered a powerful addition to FDG-PET, providing additional diagnostic specificity and important biological information that could be useful in predicting prognosis, planning treatment, and monitoring response.