DAX1 gene (Xp21) expression is involved in the development of the hypothalamo-pituitary-gonadal and adrenal axes, and acts as a negative regulator of steroidogenesis. Mutations of this gene determine adrenal hypoplasia congenita (AHC) and hypogonadotropic hypogonadism. We report the case of a 9-month-old boy referred for the study of macrogenitosomia and pubic hair development. He had presented acute adrenal crises in the neonatal period and, later, a clinical picture of peripheral precocious puberty. A mutation in the DAX1 gene was found (Trp291Arg) and a diagnosis of AHC was made. Replacement doses of hydrocortisone (HC) (10 mg/m2/day) failed to produce a feedback inhibition of adrenocorticotropic hormone (ACTH), and testosterone levels remained high. Testosterone and ACTH values normalized after HC was progressively increased to 18 mg/m2/day. In conclusion, peripheral precocious puberty in patients with DAX1 gene mutations appears to be secondary to the stimulus exerted by ACTH on melanocortin receptors in Leydig cells and to the overexpression of testicular steroidogenesis activators by the loss of transcriptional repression.