Positron emission tomography (PET) with [18F]-fluoro-3'-deoxy-3'-L-fluoro-'thymidine (FLT) provides information on cellular proliferation activity and has been used in malignant brain tumors. FLT-PET is sensitive in detecting malignant glioma due to the very low background uptake in the normal brain tissue. However, the ability to detect low grade glioma and tumor extension appears to be lower for FLT than for methionine (MET). Recent studies have found a correlation between WHO tumor grade and FLT uptake in gliomas. Our recent experience demonstrated a significant difference in the maximum standardized uptake value (SUVmax) of FLT between grade II (0.27 +/- 0.06, n=6) and grade IV (2.18 +/- 0.93, n=10) glioma (P < 0.0001), and grade III (0.70 +/- 0.45, n=7) and grade IV glioma (P < 0.001). FLT uptake correlated significantly better with the Ki-67 index (r = 0.86, P < 0.0001) than did MET uptake in gliomas. Studies have also reported the use of FLT-PET to investigate the effectiveness of therapy and prognosis of glioma. Increased FLT accumulation is also observed in other brain tumors including malignant lymphoma and metastatic brain tumor. Further studies are necessary to validate the usefulness of FLT-PET in brain tumor diagnosis and treatment.