The important role of biotin in human physiology has been highlighted by the recognition of two newly discovered human inborn errors of the metabolism of biotin. The molecular defect in the neonatal-onset disease is in the enzyme holocarboxylase synthetase. The defect in the later infantile-onset disease is in the enzyme biotinidase. Both disorders present with impressive clinical manifestations involving the skin and hair. In the neonatal disease, alopecia totalis is associated with a bright red scaly total body eruption. In biotinidase deficiency, the alopecia is more patchy and the skin lesions resemble acrodermatitis enteropathica. Both disorders are complicated by recurrent episodes of life-threatening acidosis and massive ketosis.