In a sepharad family from Morocco, 2 children out of 4 had congenital adrenal hyperplasia with 11 beta-hydroxylase deficiency. The index case had a very severe genital masculinization (Prader V), diagnosed at 18 months of age. For the fourth pregnancy, dexamethasone was given from the 8th to the 24th week of amenorrhea, in order to prevent the virilization of a female fetus. The treatment was interrupted for 5 days, at the 20th week, because of amniocentesis. As the amniotic steroid concentrations were normal, the treatment was stopped. Nevertheless the neonate also has severe virilization of the external genitalia and postnatal hormonal studies confirmed the diagnosis of congenital adrenal hyperplasia due to 11 beta-hydroxylase deficiency. The reasons for the failure of the treatment are analyzed.