100 tumours of the human nervous system were investigated by means of immunohistochemistry in order to determine the expression of epidermal growth factor receptor (EGFr) and the proliferative activity as evaluated by demonstration of the proliferation-associated Ki-67 antigen. Epidermal growth factyr receptor immunoreactivity was present in 79% (23/29) of the high-grade malignant gliomas examined but in only 9% (2/22) of the low-grade gliomas. Besides the gliomas, EGFr-expression was detectable in smaller amounts in most (13/15) meningiomas, in one anaplastic neurinoma and in individual tumour cells of one medulloblastoma. In addition, EGFr-expression was found in 50% (6/12) of metastatic carcinomas. Seven of eight medulloblastomas, two cerebral primitive neuroectodermal tumours (PNETs), three benign neurinomas, one ganglioneuroma, one metastatic intracerebral malignant melanoma, three spinal plasmacytomas and one immunocytoma showed no detectable EGFr-expression. Our results indicate that (1) the expression of EGFr in human tumours of the nervous system depends on the histological tumour type and (2) in the glioma group is related to the grade of malignancy. A close correlation between EGFr-expression and proliferative activity as evaluated by Ki-67 staining could not, however, be established.