Background: The aim of the study was to evaluate the prevalence, clinical characteristics, and diagnostic importance of nystagmus in children with brain tumours. Methods: A nation-wide retrospective review of all children diagnosed with a brain tumour between January the 1st, 2007 and December 31st, 2017, in Denmark. Data is based on information from the Danish Childhood Cancer Registry, hospital records from paediatric- and ophthalmological departments, and records from private ophthalmologists. Results: Nystagmus was observed in 13.7% (60/437) of children with a brain tumour. In 50/60 children (83.3%) nystagmus was an incidental finding at the clinical examination and only in 10/60 children (16,7%) were nystagmus noticed by patient/caregivers prior to the clinical examination. In 38/60 children nystagmus was observed before the brain tumour diagnosis, most often (16/38, 42%) the same day as the diagnosis was made. In 22/60 children nystagmus was found after the brain tumour diagnosis (prior to any treatment) with a median of four days (range 0-47) after the brain tumour diagnosis. Nystagmus was most commonly binocular (56/60, 93.3%) and gaze-evoked (43/60, 71.7%). The median number of additional symptoms and/or clinical findings was five (range 0–11). Conclusion: Nystagmus is frequent in children with brain tumours and is typically accompanied by other symptoms and clinical signs. However, nystagmus is often first recognized by the ophthalmologist late in the time course. Therefore, raising awareness of the importance of looking for nystagmus in children with unspecific neurological symptoms might contribute to increased suspicion of brain tumour and thereby faster diagnosis.