The monoclonal antibody Ki-67 recognizes a nuclear antigen which is expressed during the G1, S, G2 and M phases of the cell cycle. Immunostaining of frozen biopsy material thus presents a convenient and rapid method for the estimation of the growth fraction in human neoplasia. This report summarizes the results obtained in 178 neurosurgical biopsies. The highest incidence of Ki-67 positive nuclei was observed in 8 metastatic carcinomas (mean: 20%) and in 53 cases of glioblastoma multiforme (10%). Glioblastomas showed considerable variation, the fraction of stained nuclei ranging from 1 to 28%. For astrocytomas and oligodendrogliomas we found a close correlation between the Ki-67 index and the histological grade in agreement with known biological behaviour. In anaplastic gliomas (WHO grade III) the upper limit was 11%, in grade II gliomas, 6%. Juvenile pilocytic astrocytomas and pituitary adenomas showed mean staining indices of approximately 1%. In 13 meningiomas the mean Ki-67 index was 1% but rose to 5% in recurrent and anaplastic meningiomas. Assessment of the growth fraction in human brain tumours by immunostaining with Ki-67 could become an important tool in the prediction of the biological behaviour of nervous system neoplasms and the planning of adjuvant therapy.