Beta-ketothiolase deficiency (mitochondrial acetoacetyl-CoA thiolase, MAT or T2 deficiency) is a rare autosomal recessive disorder of isoleucine and ketone body metabolism due to acetyl-CoA acetyltransferase-1 (ACAT1) gene mutations. The disease is characterized by recurrent episodes of ketoasidosis which starts with vomiting and followed by dehydration and tachypnea. Here, we present a patient who was admitted to the hospital with severe acidosis and dehydration because of vomiting induced by protein rich nutrient and was diagnosed with MAT deficiency. 3-hydroxy-butyric acid, acetoacetic acid and 3-hydroxy-iso-valeric acid levels were significantly increased and tiglyglycine as trace amount in the urine organic acid analysis of the patient. Genetic analysis for ACAT-1 showed compound heterozygosity for the mutations c.949G > A (p.D317N) and c.951C > T (p.D317D), which both are known to cause exon 10 skipping and to be pathogenic missense mutations.