Suspicion of hereditary disorders of the intermediary metabolism must be aroused by every neonatal disease associated with a neurological affection without an apparent cause, with ketonuria, metabolic acidosis or hypoglycaemia. These disorders can be suspected clinically by common laboratory examinations. The correct diagnosis is important, as some of these diseases respond well to early treatment. The diagnosis is of fundamental importance also in those hereditary metabolic diseases which are so far incurable (prognosis of a future pregnancy). Based on their experience with 218 neonates, the authors differentiate between five groups of these diseases. Their characteristic may be a guideline for initiation of adequate therapy and for special examinations.