MIB-1 immunohistochemistry was carried out on a retrospective biopsy series of epithelial choroid plexus neoplasms in order to assess the proliferation rate of tumour cells. The material included 14 cases of papilloma (WHO grade I) and five cases of carcinoma (WHO grade III). There was one recurrent tumour in the papilloma group, while three patients with carcinoma experienced recurrence. Choroid plexus obtained at autopsy from paediatric and adult patients with unrelated diseases served as control. The average MIB-1 labelling index of choroid plexus papillomas was 3.7%, while that of carcinomas was 14%, and that of normal choroid plexus was 0.02% to 0.06%. The age and sex of the patients as well as the tumour localization were not found to influence MIB-1 reactivity. The labelling index of recurrent lesions was not significantly different from that of the corresponding primary tumours. High MIB-1 labelling indices were associated with less favourable post-operative outcome in choroid plexus carcinomas and in one papilloma with atypical histology. However, most tumours with atypical histological features did not exhibit distinctive MIB-1 labelling indices. Analysis of growth fraction by MIB-1 immunohistochemistry may prove a useful ancillary method for assessing the malignant potential of choroid plexus neoplasms.