The clinical history and the neuroradiological findings have been reviewed for 5 patients with biotinidase deficiency. Patients were diagnosed in the UK, where neonatal screening for this disorder is not done. The age at presentation ranged from 4 weeks to 5 months and the median interval between presentation and diagnosis was 5.5 months. The main abnormalities on cerebral imaging were leukoencephalopathy and widening of the ventricles and extra-cerebral CSF spaces. White matter abnormalities included delayed myelination but, in some patients, the increased signal was too great to be explained just by failure of myelination. Subtle subcortical changes were the only abnormality in one patient. Follow-up studies after treatment with biotin showed improved myelination; in one case, this was accompanied by normalisation of the CSF spaces but another patient showed progressive atrophy and cystic degeneration. Most of these patients have neurological sequelae. Biotinidase deficiency should be excluded in all patients with unexplained neurological problems. Neonatal screening provides the best chance of a good outcome.