The close genetic linkage between the gene for congenital adrenal hyperplasia due to 21-hydroxylase (21-OH) deficiency and HLA genes allowed us to use the polymorphism of this system as a marker of the disease. HLA genotyping can be performed by using restriction enzyme fragments hybridized with specific probes instead of serologic methods. In seven pregnancies at risk for 21-OH deficiency, a first trimester prenatal diagnosis has been performed by determining the fetal genotype by linkage analysis of DNA from chorionic villi using HLA class I and class II probes. In four of these pregnancies, determination of 17-OH progesterone in first trimester amniotic fluid afforded a complementary approach to the diagnosis.