Mitochondrial acetoacetyl-CoA thiolase, is an enzyme in isoleusin catabolism and ketone body metabolism. The name ‘’beta-ketothiolase deficiency’’ is usually used for the defect of this enzyme. This disorder is clinically characterized by ketoasidotic episodes, sometimes accompanied by unconsiousness. In this article, a 14 months-old infant, diagnosed in the first ketoasidotic episode, was reported because of rarity of the disease and we should consider it in the differential diagnosis of ketoasidotic episodes. ©2004, Fırat Üniversitesi, Tıp Fakültesi