A seven-year-old boy was previously treated for the primarya posterior fossa tumour, medulloblastoma, with extensive central nervous system metastases including leptomeningeal and intrathecal spinal disease; methylation profiling confirmed a Group 4 tumour. At initial presentation a chemotherapy approach was preferred, due to both his young age and extent of disease; this achieved complete radiological and cytological remission prior to consolidation with high-dose chemotherapy and autologous stem cell rescue. He then experienced an asymptomatic localised posterior fossa relapse on surveillance imaging, treated with complete surgical resection, craniospinal irradiation and maintenance chemotherapy. This chemotherapy was interrupted due to poor count recovery following irradiation, and a bone marrow aspirate and trephine were performed which excluded metastatic medulloblastoma or secondary leukaemia. Alternative maintenance with temozolomide was well tolerated. Unfortunately, end-of-treatment MRI imaging of the neuro-axis revealed an asymptomatic new small enhancing intracranial lesion. An early repeat MRI was performed six weeks later which showed minor progression of the intracranial disease and no intrathecal metastases, but new low T1 signal in multiple vertebral bodies with sparing of T3 and T7 vertebrae (arrows; left image) compared with the imaging performed just six weeks previously. Full blood count revealed Hb 97 g/l, WCC 7.7 x109/l, neutrophils 5.4 x109/l and platelets 204 x109/l. In view of the radiological appearances, bone marrow aspirate and trephine were performed from the posterior iliac crest. Aspirate revealed heavy infiltration with clusters of non-haematopoietic cells, characterized by high nuclear:cytoplasmic ratio, open chromatin and agranular, pale basophilic cytoplasm with vacuolation (right upper image). Trephine immunohistochemistry demonstrated positive staining for synaptophysin, CD56, Neu-N (right lower image), retained INI1 and negative CD99 and desmin, confirming medulloblastoma. Spread of medulloblastoma to the bone marrow is a very rare event. In this case, despite an unremarkable full blood count, radiological changes in the spinal column correlated with easily identified disease in aspirate and trephine samples taken from the posterior iliac crest suggesting widespread marrow infiltration were confirmed by bone marrow examination. Early identification of extracranial metastasis afforded the family and clinicians the opportunity to make informed choices regarding ongoing management.