Improvements in brain tumor therapy have paralleled advancements in brain tumor imaging. Thus, stereotactic surgery was enabled by computed tomography scanning and localized radiotherapy by studies that confirmed the relationship between radiologic images and the actual tumor location. The neurooncologist seeks techniques for the noninvasive identification of malignant foci within otherwise benign brain tumors. Recognizing that fully 40% of patients with nonenhancing brain masses harbor malignancies, neurooncologists have sought noninvasive techniques to separate areas of quiescent tumor from those with aggressive potential. The neurosurgeon in the modern era depends heavily upon imaging techniques to select appropriate biopsy sites. Similarly, the therapist often designs radiation simulation fields to provide boost. In this issue of Cancer, Slizofski and colleagues' report on the use of thallium 201-chloride (TI-201) single photo emission computed tomography (SPECT) for grading brain tumors. Thallium enters the brain only in areas with a disrupted blood brain barrier, and then (like potassium) is concentrated in viable tissue. The authors demonstrate that thallium uptake differentiates between benign and malignant tissue. Uptake ratios (tumor:normal brain) above 1.5 characterized viable malignant tumor either before or after therapy. Three instances of discordant results occurred among the 40 scans performed, Single photo emission computed tomography, "the poor man's position emission tomography (PET) scan" could have an important role in evaluating brain tumors. The radiopharmaceutical TI-201 routinely is available in all nuclear medicine departments and can be imaged easily with standard equipment. This simple imaging technique may help to reduce sampling errors From the Services of *Neurology, tRadiology (Nuclear Medicine) and the SMorphometry Laboratory of the Massachusetts General Hospital,