Aims: To evaluate the prognostic value and clinical utility of Ki67 tumour cell proliferation index in anaplastic oligodendroglial tumours (AOT).Methods And Results: We performed anti-Ki67 immunostaining (MIB-1 antibody) of formalin-fixed and paraffin-embedded tumour tissue specimens of 128 patients with newly diagnosed AOT that were treated in a randomized Phase III trial. Ki67 index was assessed by three independent observers and was correlated to clinical, histopathological and molecular features (including 1p/19q co-deletion, epithelial growth factor receptor gene (EGFR) amplification, isocitrate dehydrogenase (IDH1) mutations, O6-methylguanine-DNA methyltransferase gene (MGMT) promoter methylation, and patient survival times. Intra- and inter-observer agreement of Ki67 index assessment was excellent. Univariable analysis (n = 79) showed that patients with a low Ki67 index had significantly more favourable progression-free survival (PFS) (P-value = 0.004, log-rank test) and overall survival (OS) (P-value = 0.003, log-rank test) than patients with a high Ki67 index, respectively. On multivariable analysis (n = 43), Ki67 index showed no independent association with PFS or OS.Conclusions: In AOT the Ki67 index has a strong prognostic impact on univariable analysis, but no independent influence on multivariable analysis. However, further prospective studies including larger numbers of cases and standardized evaluation of Ki67 index in conjunction with other relevant prognostic parameters are needed to draw definitive conclusions.