A mother carrying a fetus affected with 21-hydroxylase deficiency received prenatal treatment with dexamethasone (0.5 mg, tid, p.o.) started from the very beginning of the 8th week of gestation. Prenatal diagnosis had to rely on amniocentesis with karyotyping and steroid hormone determination, because HLA and DNA data from the deceased index case or direct molecular genetic techniques were not available. The pre- and postnatal diagnosis of 21-hydroxylase deficiency was based on mass spectrometric determination of 17-hydroxyprogesterone. Dexamethasone was discontinued for 5 days prior to amniocentesis. Monitoring of cortisol, dehydroepiandrosterone-sulphate and oestriol in maternal plasma revealed suppressed maternal and fetal adrenal glands throughout pregnancy. Plasma dexamethasone levels confirmed excellent maternal compliance. At term, an eutrophic girl with normal female genitalia was delivered. The diagnosis of 21-hydroxylase deficiency and salt loss was confirmed postnatally. Regarding the side-effects of dexamethasone, the benefit/risk ratio was in favour of prenatal dexamethasone therapy.