Thal1ium-201 (201TI) SPECT Studies were performed on a normal volunteer and 12 patients with intracerebral lesions: 3 patients with gliomas, 3 patients with meningiomas, 1 patient each with metastatic tumor, brain abscess, and cerebral infarction, and 3 postirradiation patients. (2 with metastatic tumors, I with lymphoma). A 201T1 index, based on the ratio of 201T1 uptake in the brain lesion Versus the homologous contralateral brain, was calculated and compared with tumor histology and CT/MRI findings. The SPECT 201T1 Scan showed minimal uptake of tracer in a normal brain. There was substantial uptake of 201T1 in high-grade gliomas (index〉l.5) with little uptake in low-grade lesions. A previously irradiated patient with recurrent astrocytoma, in whom MRI study was unable to distinguish tumor recurrence from necrosis, showed the lesions with high 201T1 indices in both hemispheric regions (2.50/1.93), suggesting tumor recurrence. Two meningiomas and a metastatic tumor showed varying degrees of 201T1 uptake (index 1.71-8.15), revealing that 201Tl uptake is not exclusive to high-grade gliomas. In 2 postirradiation patients with metastatic tumors, no abnormal 201T1 uptake was found in the cerebral lesions, shortly after the initiation of radiation therapy or despite the persistence of enhancing lesions-though improved-on MR images, suggesting that 201T1 uptake may reflect the metabolic and possibly clonogenic activities of tumors and the brain 201T1 SPECT imaging might be valuable for the evaluation of tumor responsiveness to the therapy and for early detection of tumor recurrence. A patient with brain abscess on antibiotic treatment, showing increased uptake of 201T1 in the resolving lesions(index 2.87/1.52) is discussed. In a patient with cerebral infaraction, there was no abnormal uptake of 201T1 in infarcted tissue. When using a threshold index of 1.5, correlation rate between 201T1 uptake and contrast enhancement of the cerebral lesions on CT/MRI was 73%(8/11). In conclusions, the brain 201T1 SPECT imaging may be useful for assessment of tumor response to the therapy and to predict low or high-grade lesions.