The clinical significance of Thallium-201 single-photon emission computerized tomography (Tl-201 SPECT) in the evaluation of viability of gliomas was studied comparatively with histological examination of tumor tissue using Ki-67 and proliferative cell nuclear antigen (PCNA) monoclonal antibody. The relationship between radionuclide uptake of Tl-201 in tumor specimens and labeling indices of special staining using Ki-67 and PCNA monoclonal antibodies were also studied. The population studied consisted of 17 patients with glioma. Tl-201 indices obtained from early and delayed images and its washout rates were used for quantitative analysis of Tl-201 SPECT findings. Tl-201 indices showed high values according to the histological malignancy of gliomas. Radionuclide uptake of Tl-201 in the tumor specimens were also high in those with high labeling indices of Ki-67 and PCNA. The lesions with marked Tl-201 uptake on early and delayed images had numerous Ki-67 and PCNA positive-stained cells. The lesion with low Tl-201 washout rate therefore reflected well the more viable lesion in the tumor tissue. It is concluded that the viability of gliomas including anaplastic changes and response of gliomas to the treatment can be detected by serial study with Tl-201 SPECT.