The correlation of magnetic resonance imaging (MRI) with histopathological findings was analysed in 26 patients with untreated cerebral gliomas. In low-grade gliomas, T2-weighted images demonstrated relatively homogeneous high-intensity lesions involving both the grey and the white matter. In high-grade gliomas, especially grade IV, T2-weighted images demonstrated prominent heterogeneity in signal intensity, which consisted of a hyperintense "core", less hyperintense or normal intensity "rim" and surrounding finger-like areas of high intensity. Marked and irregular contrast enhancement was evident in all but one case of these high-grade gliomas in which gadolinium-DTPA was used. Histological examination revealed tumour cells extending as far as the borders of the high-intensity areas shown on T2-weighted images in both high- and low-grade gliomas, but in 5 of 8 low-grade and 4 of 18 high-grade gliomas, isolated tumour cells extended beyond the hyperintense areas shown on T2-weighted images.