Abstract INTRODUCTION Adult medulloblastomas are clinically and molecularly understudied due to their rarity. METHODS We studied a cohort of 101 adult medulloblastomas grouped by Nanostring assay. We performed targeted sequencing on exonic regions of 69 genes implicated in medulloblastoma, and Sanger sequencing for TERT promoter hotspot mutations. RESULTS Median age was 27 (range 19-63), and 9.5% were metastatic at diagnosis. SHH accounted for 50% of cases. Unlike previous studies, Group 3 comprised a significant proportion (14%) of adult medulloblastomas, comparable to WNT (19%) and Group 4 (18%). Median follow-up was 51.1 months. Median OS and PFS were 102 and 99 months respectively. In contrast to paediatric medulloblastomas, molecular groups had no prognostic impact in adults, for both OS (p=0.956) and PFS (p=0.162). Most frequently mutated genes were TERT (including promoter, mutated in 35% cases), chromatin modifiers KMT2C (32%) and KMT2D (31%), TCF4 (31%), PTCH1 (26%) and DDX3X (24%). Adult WNT patients showed enrichment of TP53 mutations (6/15 WNT cases), and 3/6 TP53-mutant WNT tumours were of large cell/anaplastic histology. 5 WNT cases harboured concurrent CTNNB1 and PTCH1 mutations. Adult SHH medulloblastomas had frequent upstream pathway alterations (PTCH1 and SMO mutations) and seldom downstream alterations (SUFU mutations, MYCN amplifications). TERT promoter mutations were found in 71% of adult SHH patients, and were restricted to this group. Adult Group 3 patients lacked hallmark MYC amplifications, but had recurrent mutations in KBTBD4 and NOTCH1. Adult Group 4 patients had recurrent mutations in TCF4 and chromatin modifier genes. Overall, amplifications of MYC and MYCN were rare (3%) in adult medulloblastomas. CONCLUSIONS We identified distinct clinical and mutational characteristics of adult medulloblastomas that will inform their treatment and risk stratification. Importantly, molecular groups were not prognostic in adult medulloblastoma, TP53 mutations were enriched in adult WNT, and MYC amplifications were absent in adult Group 3.