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3-hydroxy-3-methylglutaric aciduria (OMIM 246450) is a rare autosomal recessive inborn of metabolism due to the deficiency of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) lyase, an enzyme involved both in the ketogenic pathway and leucine catabolism. Acute decompensations present with lethargy, cianosis, hypotonia, vomiting and metabolic acidosis with hypoketotic hypoglycemia. We report the case of a 3 days male with sudden hypoglycemic crisis initially misdiagnosed as a sepsis. HMG-CoA lyase deficiency was achieved through acyl-carnitines profile (showing a typical increasing of 3-hydroxy-isovaleryl and 3-methylgluraryl carnitines) and urinary organic acids analysis (disclosing elevation of 3-hydroxy-3-methylglutaric, 3-methyl-glutaconic, 3-methylglutaric and 3-hydroxyisovaleric acids). This case underlines the need of suspecting such inborn metabolic disorder in cases with hypoglycemia and metabolic acidosis. Acyl-carnitine and urinary organic acids profiles are essential to achieve a prompt diagnosis of treatable metabolic disorders in order to prevent their acute crisis with serious or even fatal consequences.