This is a report of an infant with 3-hydroxy-3-methylglutaryl-CoA-lyase deficiency. During infancy vomiting and feeding difficulties had been found repeatedly. Nutrition consisted in a mixture of one third of milk diluted with two thirds of water. Shortly before admission a change to whole milk had taken place. The one year old girl was admitted to the hospital because of vomiting and diarrhoea. Liver was enlarged and consciousness reduced. There was acidosis, hypoglycemia and an elevation of the transaminases. The typical pattern of organic acids in urine and dicarboxylic aciduria was pathognomonic. The diagnosis was proved by a deficiency of 3-hydroxy-3-methylglutaryl-CoA-lyase in fibroblasts. A protein and fat restricted diet, but also carnitine supplement was introduced. With the exception of severe hypoglycemia seizures on one morning the more than 2 1/2 year old girl is developed normally. It is of importance for future to avoid hypoglycemia after fasting, infections diseases or stress. The possible impairment of pancreatic function even in a normal clinical condition with normal values of transaminases is especially pointed out.