The girl was typically treated with insulin in constant iv infusion for 37 hrs. After 8 hrs she started to eat carbohydrate exchanges and take sc insulin injections. At the 4th day of hospitalization unexplained recurrent vomiting appeared. Only then the patient's mother mentioned about 3MCC deficiency identified by NBS, with no further specific recommendations. Urinary organic acid analysis by GC/MS confirmed the diagnosis. So additionally to carbohydrate, fat/protein counting, low-protein diet based on special formula and L-carnitine were introduced. Until now three hospitalizations occurred: 1st for implementation of personal insulin pomp, 2nd due to ketotic acidosis caused by technical problem with pomp/ catheter, 3 rd metabolic decompensation in the course of rotaviral infection, which required 6-day-long 10% glucose and insulin iv infusions. Current therapeutic recommendations are: low-protein ( Background: 3-Methylcrotonyl-CoA carboxylase deficiency is an inherited metabolic disorder biochemically characterized by high tissue accumulation and urinary excretion of 3-hydroxyisovaleriate, 3methylcrotonylglycine (3-MCG) and 3-hydroxyisovaleryl-carnitine. Clinically, patients present severe brain abnormalities and neurologic dysfunction, whose pathomechanisms are still unclear.