The manifestations of Inborn Errors of Metabolism (IEM) with an acute presentation are extremely unspecific, which makes its diagnosis a task of laboratorial interpretation. At the same time, the early diagnosis of many of these IEM allows quite satisfactory therapeutic interventions, followed by favorable clinical evolutions. To make the work of pediatricians and neonatologists easier in the diagnosis and immediate management of these conditions with an acute onset, a flowchart of procedures is suggested. The different steps suggested are justified and complemented by adjacent comments, which are restricted to the more serious and common manifestations of these disorders, like metabolic acidosis, hyperammonemia and hypoglicemia. Finally, the reading of specific and more detailed literature is suggested for the understanding of the different families of specific IEM.