A guideline for early diagnosis of metabolic disorders affecting central nervous system during neonatal and early infancy was presented. Clinical manifestations associated with inborn errors of metabolism in the neonatal period are poor feeding, vomiting, diarrhea, abnormalities in muscle tonus, dyspnea, convulsion, coma and so on, and these are not specific to each disorder. However, such symptoms or signs as described below have often intimate relation to metabolic disorders: (1) previous children died of undetermined causes during early infancy; (2) complication of sepsis; (3) onset in the early neonatal period; (4) developmental and growth retardation. When newborns and infants have these symptoms or signs, we should start simple screening studies immediately for metabolic disorders, including CBC, hepatic function tests, blood glucose, lactate, pyruvate, ketone bodies, ammonia, blood gas analysis, urinalysis (including non-glucose reducing substance tests and FeCl3 reaction) and so on. As for CBC, we have to make our own effort to find spherocytosis and vacuoles in lymphocytes. Family history, especially the mother's personal history, is indispensable. During physical examinations, we must pay attention to facial appearance, skin color, macroglossia, hair abnormalities, peculiar odor of the urine and hepatosplenomegaly. When abnormality is found in these clinical signs or simple laboratory examinations, we should not hesitate to start dietary treatment even if special examinations for differential diagnosis are on the way.