Inherited hyperammonemia disorders are caused by specific enzymatic defects in the urea cycle or in metabolic pathways related to it. These disorders can be divided into the following groups: deficiencies of urea cycle enzymes, transport defects of dibasic amino acids, organic acidemias, defects in beta-oxidation of fatty acids, transient hyperammonemia of the newborn-probably a not genetically determined disorder. Manifestation of the mentioned disorders includes elevated serum ammonia level resulting in altered level of consciousness and/or persisted vomiting. Occurrence of irreversible neurologic sequelae depends mostly on the extent of hyperammonemic period. Differential diagnosis includes blood gas, anion gap, plasma amino acids analysis and urine organic acids analysis. In some cases specific tissue enzymes activity measurement is necessary. Dialysis, sodium benzoate, sodium phenylacetate and arginine are used in the treatment of acute hyperammonemia. In addition oral or rectal neomycin and/or lactulose can be used, which reduces intestinal ammonia production.