The biochemical, dermatological and neurological motor disorders of biotinidase deficiency (multiple carboxylase deficiency) show a dramatic response to pharmacological doses of biotin. This condition is characterised by the accumulation of biocytin and depletion of biotin. Neuromuscular function returns to normal with the reversal of the characteristic organic acidaemia. It would appear that the optic and auditory nerves or their related neurological structures may suffer damage from the excess biocytin and deficient biotin. Despite reversal of the dermatological and psychomotor abnormalities children are likely to be left with auditory and/or visual handicaps if diagnosis and treatment is delayed beyond the first year of life. Treatment with biotin was commenced 6, 18, and 13 months after onset of symptoms. Two children subsequently were found to have visual impairment (acquired retinal dysplasia) and two had sensori-neural deafness. In one patient both defects were present.