The proliferative activity of 133 human tumors of the nervous system was investigated by means of immunohistochemistry using the monoclonal antibody Ki-67 in order to evaluate the usefulness of this method for histopathological tumor grading. Ki-67 recognizes a proliferation-associated nuclear antigen present in human cells during all active phases of the cell cycle but absent in the G0 phase [Gerdes J, Schwab U, Lemke H, Stein H (1983) Int J Cancer 31:13-20]. In 28 WHO grade I and II gliomas of all major types Ki-67 indices were generally low with mean values ranging from less than 1% in pilocytic astrocytomas to 4.2% in grade II oligodendrogliomas. Individual cases of grade II astrocytomas and oligodendrogliomas had, however, values up to 8.5%. In 13 primary anaplastic gliomas of WHO grade III consistently higher statistical means were obtained with values ranging from 8.6% for anaplastic astrocytomas to 14.2% for anaplastic mixed gliomas. Interestingly, 18 WHO grade IV glioblastomas demonstrated a mean value of only 7%, which is probably due to the pronounced phenothypic heterogeneity in this tumor group. This heterogeneity results in enormous intra- and intertumor variability in Ki-67 indices (range less than 1%-22.1%). Investigation of 17 recurrent gliomas revealed mean values for Ki-67 ranging from 1.7% for three WHO grade II astrocytomas up to 48.5% obtained in two highly anaplastic recurrent astrocytomas corresponding to WHO grade IV. Other tumors of the nervous system evaluated included 9 medulloblastomas (mean 17.9%, range 5.0%-42.0%), 17 benign meningiomas (mean 1.1%, range 0%-5%), 15 metastatic carcinomas (mean 16.5%, range less than 1%-46.0%), and individual tumors of various types. Our results indicate that Ki-67 immunohistochemistry can add useful additional information for histopathological grading which, by supplementing and refining the traditional WHO grading system, might lead to a better assessment of the biological behaviour of human tumors of the nervous system.