Biotinidase deficiency is a disorder of biotin metabolism. Patients are classified as having either a profound or partial deficiency based on measurement of biotinidase activity in serum. Clinical features of profound untreated biotinidase deficiency include seizures, ataxia, hypotonia, developmental delay, alopecia, hearing loss, eye problems, skin rash, lactic acidosis and ketosis. Onset of symptoms usually occurs by several months of age but may occur during late childhood or adolescence. Partial biotinidase deficiency may exhibit any of the above symptoms but the symptoms are usually milder and may only occur during periods of metabolic stress. The symptoms of biotinidase deficiency can be prevented by administration of oral biotin making this disorder highly amenable to newborn screening programs in the U.S. and worldwide. However, once the eye, hearing problems, and developmental delay occur, they may be irreversible.