Adrenal hypoplasia congenita (AHC) is a rare disease that can be caused by many abnormalities, including an X-linked form. Mutations in the DAX1 gene have been assigned as the genetic cause of AHC. We describe here three siblings with AHC, clinically presented at different ages, two in the neonatal period and one oligosymptomatic during infancy. Molecular analysis was able to detect a novel mutation in exon 1 of the DAX1 gene, consisting of a transition of C to T at position 359, determining a stop codon at position 359 (Q359X). The mutated gene encodes a truncated protein missing a large portion of the ligand-binding domain (C-terminal domain). The recognition of the disease in the index case suggested the diagnosis in the other siblings. Interestingly, the same mutation is presented with different phenotypes, suggesting that first-degree family members of patients with DAX1 mutations should be carefully evaluated routinely.