A patient with biotinidase deficiency was studied in whom the first admission to hospital for acidosis occurred at 5 years of age. Sensorineural abnormalities of the optic and auditory nerves antedated diagnosis and treatment with biotin, and these sensory losses did not resolve with treatment. The other clinical manifestations of the disease were highly responsive to biotin. Biotinidase was assayed using 14C-labeled natural substrate. The activity in the patient approximated 1% of the control level.