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Beta ketothiolase deficiency is a rare autosomal recessive disorder of isoleucine and ketone body metabolism characterised by unexplained episodes of ketoacidosis which is often triggered by infections, prolonged fasting and large protein load. A 6 months old boy was admitted to our hospital with history of convulsions, lethargy and loss of consciousness, dehydration with metabolic acidosis, hypoglycemia and ketonuria. β- ketothiolase deficiency was confirmed by urine Gas Chromatography –Mass Spectrometry. This case is being reported because of rarity of the disease and should be considered as a differential diagnosis in ketoacidotic episodes.