This article describes the metabolic investigations to be applied in any clinical situation consistent with a late acute form of inborn error of metabolism: unexplained coma with or without focal neurological manifestations, recurrent vomiting with lethargy, episodes of ataxia with or without behaviour disorder, fits of psychiatric troubles. In each of these situations, careful medical history is of major importance searching for previous clinical manifestations such as episodes of coma, ataxia or vomiting, anorexia, failure to thrive, developmental delay, all very suggestive of metabolic disorder. The association of neurological symptoms and abnormal hepatic tests is also of great value and must not lead to the diagnosis of Reye's syndrome without considering a metabolic defect of fatty acid oxidation, urea cycle, respiratory chain, or Wilson's disease. When looking for an etiological origin, it is mandatory to collect all the biological information at the same time, also knowing that metabolic abnormalities may be mild and transitory, and that many of them are non specific (metabolic acidosis, hyperlactacidemia, hyperammonemia, hepatic tests disturbances) being encountered in collapsus, shock and multiple organ failure syndrome.