After initial radiotherapy for an intracranial malignant glioma, the majority of patients return at a later date with a recurrent, enhancing mass on computed tomography or magnetic resonance imaging. This mass represents either recurrent tumor, radionecrosis, or a combination of the two. The relative proportion of live versus dead tumor cells is difficult to determine from surgical specimens of another biopsy, although this has been the preferred method of assessing such "failed" patients. Recently, attention has turned to tomographic images of metabolic markers, i.e., positron emission tomography and thallium-201 (Tl-201) single photon emission computed tomography, as noninvasive methods of assessing relative tumor viability. To assess whether Tl-201 uptake in vivo can be used as a prognostic indicator in patients with glioblastoma multiforme, we measured the ratio of Tl-201 uptake in tumor to Tl-201 uptake in myocardium (T/C ratio) in 16 patients at the point of treatment "failure" and followed all the patients until they died. All patients died of neurological causes, and 11 of the 16 patients had documented viable tumor recurrence. There was a significant negative correlation between the T/C ratio at failure and the time interval between failure and death (r = -0.602, P = 0.014). Patients with T/C ratios of less than 0.3 lived an average of 13 months, whereas patients with T/C ratios of more than 0.3 lived an average of only 4 months.(ABSTRACT TRUNCATED AT 250 WORDS)