Congenital adrenal hyperplasia (CAH) is the common name of a constellation of diseases that impair cortisol synthesis in the adrenal cortex. As defects in each of three steroidogenic enzymes, 21-hydroxylase, 11 beta-hydroxylase, and 3 beta-hydroxysteroid dehydrogenase, promote overproduction of adrenal androgens, affected female fetuses may be virilised. The major cause of CAH is 21-hydroxylase deficiency, the incidence of which is 1:10,000 live births in the Swedish population. Of the 10-15 children born in Sweden each year with 21-hydroxylase deficiency, 3-5 will be virilised girls who must undergo traumatic surgery of the external genitalia. This can be prevented by administration of dexamethasone to the gravida during pregnancy. Prenatal treatment was introduced in Sweden in 1985, prenatal diagnosis being based in most cases on direct mutational analysis using allele-specific PCR on DNA from chorionic villus samples. In our experience, genotype corresponds well to phenotype, and we recommend that all children with 21-hydroxylase deficiency be genotyped in order to prepare the family for rapid and reliable prenatal diagnosis and possible treatment when the next child is awaited. Since 1985, 35 women have received prenatal treatment in Sweden, six of the 35 fetuses being found to be affected females and treated until term. As compared with their older sisters, all of these six girls were characterised by no signs, or only minor signs, of virilisation, and only one required surgery because of labial fusion and recurrent urinary tract infections. As a group, the 35 infants were characterised by normal birth weight and length, and normal growth during the first year of life. Passage of developmental milestones was normal though several adverse events, both in treated mothers and infants, have been reported. Approximately one fourth of the women treated throughout pregnancy reported some side-effect (e.g., excessive weight gain, severe cutaneous striae, mood fluctuations and irritability, acne and hirsutism, or oedema). One unaffected boy, treated for seven weeks, was born with severe hydrocephalus and agenesis of the corpus callosum; two affected sisters and one unaffected girl were characterised by failure to thrive during the first year of life, but later recovered (one of the affected sisters was later diagnosed as suffering from mitochondrial disease). Although in our experience prenatal treatment with dexamethasone is effective in preventing virilisation of girls with 21-hydroxylase deficiency, some adverse events have been noted in treated infants. As it remains unknown whether these events were attributable to the treatment, it must still be regarded as experimental, and its use should be centralised and meticulously monitored until more experience has been gained.