Sequential Thallium-201 and Technetium-99m-HMPAO SPECT images were obtained from 7 patients with high-grade gliomas and 5 patients with low-grade gliomas. The histo-pathological tumor-type was verified from either biopsy specimens or resected tissue from surgery. There was an intense uptake of Thallium-201 in all high-grade gliomas. Compared to the contralateral hemisphere the mean uptake was 4.6 times higher. Low-grade gliomas showed only a marginally increased Thallium-201 uptake (1.4 times). No overlap existed between the two groups. There was no correlation between Technetium-99m-HMPAO uptake and tumor type. Thallium-201 appears to be a promising radiopharmaceutical for differentiating high- and low-grade gliomas, whereas Technetium-99m-HMPAO is not.