Our patient was diagnosed with multisystemic Langerhans Cell Histiocytosis (LCH) at 5 months of age. She received an allogeneic matched unrelated transplant for refractory haematopoietic disease. 2 years later she represented with raised intracranial pressure and an MRI confirmed the presence of a large left posterior fossa mass with homogenous enhancement and cystic areas, abnormal spectroscopy pattern, minor diffusion restriction and associated triventricular hydrocephalus. With the exception of CD1a and Langerin negativity, in contrast with initial diagnostic skin biopsy, the lesion was consistent with a diagnosis of LCH. BRAF V600E mutation was present on the cerebellar and skin sample.