A quantitative preoperative technique using thallium-201 single-photon emission computerized tomography is described which predicts whether specific gliomas are of high- or low-grade malignancy. An index, based on the ratio of thallium uptake in the tumor versus the homologous contralateral brain, was calculated and compared with tumor histology. The index in 14 patients with low-grade malignant gliomas was 1.27 +/- 0.40 in contrast to an index of 2.40 +/- 0.61 in 11 patients with high-grade malignant gliomas (p less than 0.0005). Whether gliomas were of low- or high-grade malignancy could be predicted with 89% accuracy using a threshold of 1.5. Low-grade gliomas with an index higher than 1.5 acted biologically more like high-grade tumors, and no tumor histologically classified as being of high-grade malignancy had an index lower than 1.7. This technique could help to reduce unrecognized sampling errors during needle biopsies of brain tumors, particularly of high-grade lesions classified in error as low-grade tumors due to inadequate biopsy material.