Biotinidase deficiency is a hereditary metabolic disease from the group of organic acidurias with an autosomal recessive type of inheritance. The disease is caused by mutations in the BTD gene, which encodes an enzyme biotinidase. Deficiency of biotinidase leads to insufficiency of intracellular Biotin, which is the coenzyme of four carboxylases involved in gluconeogenesis, leucine metabolism and biosynthesis of fatty acids. At infringement of function of carboxylase accumulate substrates that are toxic to the human body. Deficiency of biotinidase is manifested primarily by neurocutaneous disorders. The Central nervous system is particularly vulnerable, since the activity of biotinidase in the human brain is very low and therefore, for the normal functioning of neurons, it is necessary to receive enough biotin through the blood-brain barrier. With its deficiency, neurological disorders for a certain period may be the only sign of the disease. Symptoms can be successfully cured or prevented by the introduction of pharmacological doses of biotin. The article presents two clinical observations of young children with biotinidase deficiency, the main manifestation of which in the first clinical case were neurological disorders, in the second – respiratory disorders. To confirm the diagnosis, an enzyme diagnosis was carried out, which revealed a low level of biotinidase. The rapid and pronounced efficacy of biotin therapy with cessation of attacks, the possibility of cancellation of anticonvulsants, regression of neurological, skin and respiratory disorders.