Methylacetoacetyl-coenzyme A thiolase (MAT) deficiency is an autosomal recessive disease caused by a defect of mitochondrial acetoacetyl-CoA thiolase (T2). There is an error of isoleucine catabolism and ketone body utilization due to mutations in the acetyl-Coenzyme A acetyltransferase 1 (ACAT1) gene. We report a case of a 14 months old Sabahan boy with beta deficiency who presented with severe sepsis and ketoacidosis who subsequently recovered..