The clinical and pathological data of ten patients with gliomatosis cerebri are compared with 48 well documented cases from the literature. The most striking clinical findings were behavioural and mental changes, seizures, motor weakness and headaches. Though diagnostic techniques have gained in sophistication, the clinical diagnosis of gliomatosis cerebri remains difficult. Laboratory and radiograph tests are mostly unconclusive. Expectations that computed tomography might lead to an accurate diagnosis were not fulfilled. Histological examination disclosed a diffuse proliferation of glial elements infiltrating normal nervous tissue with destruction of myelin sheaths, but only slight damage to neurons and axons. In two cases, areas typical of oligodendroglioma were also present. Glial fibrillary acidic protein staining showed in seven cases that most of the neoplastic cells were of astrocytic origin. In addition, GFAP negative neoplastic cells with the appearance of oligodendroglia and intermediate elements between astroglia and oligodendroglia and irregularly shaped naked nuclei of unidentified nature were found. On the basis of the two-stage theory of carcinogenesis, it is suggested that this disease might be the result of propagation of initiated glial elements which have not yet undergone the process of tumor conversion.