A long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency is a hereditary disease referred to the group of disorders of the mitochondrial β-oxidation of fatty acids. The inheritance mechanism is autosomal recessive. The several main symptoms of the disease include hypoglycemia, liver steatosis, cardiomyopathy, cardiac arrhythmias, progressive muscle hypotension. Laboratory signs include a relative increase in the concentration of long-chain fatty acids, as determined by tandem mass spectrometry. Also, a characteristic feature is a low rate of free carnitine (C0), normally exceeding 20 μmol/liter. We have presented a case of a successful diagnosis and treatment of a deficiency of 3-hydroxyacyl-CoA long-chain fatty acid dehydrogenase in an infant of 8 months. There is emphasized the importance of the earliest possible verification of the diagnosis and initiation of diet therapy, which allows offsetting the manifestations of the disease. It determines the need to include diseases of the group of disorders of mitochondrial β-oxidation of fatty acids into the neonatal screening program.