Thallium-201 SPECT has shown varing uptakes in tumors, depending on their viability, the type, and degree of malignancy. The authors analyzed Thallium-201 SPECT in brain lesions quantitatively, to predict the degree of malignancy, to differentiate between high and low grade astrocytic tumor by thallium uptake, and to differentiate radiation necrosis from tumor recurrence. The thallium indices were 1.56+/-0.75 in the benign brain tumors and 2.5+/-1.13 in the malignant brain tumors(p<0.05). There was differences in thallium index between high(2.19+/-0.61) and low(1.87+/-0.81) grade astrocytic tumor, however it was not statistically significant(p=0.55). The diagnostic accuracy of thallium index in differentiating benign from malignant was 82.5% by a threshold of 1.5. The mean thallium index of radiation necrosis was lower than that of recurrence(1.33/2.22). In conclusion, Thallium-201 SPECT may be useful for the diagnosis and evaluation of brain tumors to predict the degree of malignancy.