OBJECTIVE: Olfactory neuroblastomas (ONBs) are rare malignant tumours that arise in the nasal vault. To date, the Hyams grade remains the only widely used histological grading system. However, it is based only on morphological criteria, and has not been updated since 1988. The objective of this study was to explore the prognostic potential of the Ki67 proliferation index (PI) and tumour-infiltrating lymphocytes (TILs) in ONB. RESULTS: A retrospective study was conducted on a bicentric series of 45 cases. The Ki67 PI was determined by counting at least 1000 nuclei on whole slides. TILs were evaluated with CD20, CD4 and CD8 immunohistochemical markers on whole slides. In this series, Hyams grades I, II, III and IV accounted for 13.4%, 44.4%, 20% and 22.2% of all cases, respectively. The Ki67 PI ranged from 1 to 93; the Ki67 PI was significantly higher in Hyams grade III-IV ONBs than in Hyams grade I-II ONBs (P < 0.0001). A Ki67 PI of ≥25 was associated with poorer survival (P = 0.02). TILs were present in both stromal and intratumoral compartments, but were located predominantly in the stromal component of the tumour. The numbers of intratumoral CD8+ cells/mm2222 CONCLUSIONS: Our findings suggest that the Ki67 PI and TILs could be used as prognostic markers, as a potential alternative to the Hyams grade.