MR images in 54 patients with biopsy-proven diffuse or "fibrillary" astrocytomas were analyzed and compared with the histopathologic features in order to determine which histopathologic characteristics underlie the radiographic findings in these gliomas and whether radiographic findings are more closely correlated with individual histopathologic characteristics than with histologically determined tumor grade. The MRI features studies included tumor heterogeneity, edema, mass effect, border sharpness, "anatomic invasion", contrast enhancement, hemorrhage, and the presence of flow voids, calcium and cyst formation. The histopathologic characteristics studied included cellular atypia, mitoses, cellularity, endothelial proliferation, necrosis and tumor grade. Edema (P less than 0.01), flow voids (P = 0.02) and contrast enhancement (P less than 0.01) demonstrated a direct correlation with tumor grade, but edema (P less than 0.01) and contrast enhancement (P less than 0.01) also demonstrated a significant correlation to tumor cellularity. Tumor heterogeneity was associated with the presence of necrosis (P = 0.01). Hemorrhage occurred only in high grade tumors, where it correlated with endothelial proliferation (P = 0.04).