Neonatal acute adrenal insufficiency is a rare condition. Congenital adrenal hyperplasia with 21-hydroxylase defect appears to be the most frequent cause, but the neonatal screening has improved its potential severe outcome. The other causes and the various clinical presentations have been exposed, with a special reference to the salt-wasting syndrome. Among them, the severity of X-linked adrenal hypoplasia congenita (AHC) deserves special attention. Two other causes of adrenal hypoplasia have been recently discovered, i.e. a mutation of the SF-1 gene and the syndrome IMAGe. Adrenal insufficiency secondary to ACTH deficiency is often unrecognised despite the risk of severe seizures and hypoglycaemia with brain damage. Finally, the hormonal diagnostic testing and the main therapeutic approach by corticosteroids have been indicated. The aim of this work is to focus the attention of paediatricians who examine a newborn because the risk of delayed diagnosis and fatal outcome may be limited if the clinical symptoms are soon recognized.