Beta-ketothiolase deficiency is a rare autosomal recessive disorder of isoleucine and ketone body metabolism. This disorder is clinically characterized by ketoacidotic attacks. Ketoacidosis, vomiting, and dehydration, lethargy and coma may be seen during attacks. A 9-month-old girl was admitted to our hospital with acidosis and dehydration. The patient was lethargic. Ketoacidosis was suspected because of acetone odor on her breath. Her blood glucose level was 262 mg/dL and urine ketone was (++++). Branched chain amino acid levels were elevated in her blood sample. Organic acid analysis of urine revealed 2-methylacetoacetyl-CoA thiolase deficiency. This was reported because of rarity of the disease and we should consider it in the differential diagnosis of ketoacidotic episodes.