The clinical description of a rare hereditary metabolic disease of an amino acid 3-hydroxy-3 methylglutaric aciduria is presented, which belonging to the group of organic aciduria and determined by a deficiency of the mytochondrial enzyme 3-hydroxy-3-methylglutaryl CoA lyase. The disease manifested on the third day of life with tonic convulsions with a violation of acid-base indicators. Against the backdrop of medical correction in the intensive care unit and within 36 hours, the patient’s condition was stabilized, the convulsions were stopped. Tandem mass spectrometry was performed, an elevated level of 3-hydroxy-isovaleryl-2-methyl-3-hydroxybutylarnitine was found in the blood. Determination of organic acids in the patient’s urine showed an increase in the concentrations of dicarboxylic acids, which is characteristic for the hereditary metabolic disease of 3-hydroxy-3 methylglutar aciduria. Partial analysis of the exon 5 HMGCL gene determined the mutation NM_00019 c.C392TS131L in the homozygous state, not previously described in the literature. Father and mother of the proband heterozygous state mutation is found. The family received recommendations for prenatal diagnosis based on molecular genetic diagnosis.