We present the case of a 4-day-old newborn with serious dehydration, polypnea, hypertonus and lethargy. Blood analysis showed severe metabolic acidosis with ketonemia, ketonuria and elevation of the GAP anion. Urine analysis revealed increased excretion of 2-methyl-3-hydroxybutyrate acid, tiglycine, and 2-methylacetoacetate acid. Neonatal onset of mitochondrial acetoacetyl-CoA thiolase (T2) deficiency is exceptional. Most patients have no clinical symptoms in the neonatal period. This entity should be considered in patients with acute metabolic acidosis and ketosis with normal glycemia and aciduria. The urine contains large amounts of 2-methylacetoacetate and its decarboxylation products. In the neonatal period, this inherited disorder of metabolism can produce severe hydroelectrolyte disorders in the form of a gradual process or acute episodes, which can occasionally be fatal.