The polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) profile analysis could be applied to the prenatal diagnosis of steroid 21-hydroxylase deficiency. We designed PCR primers to amplify most of the 21-hydroxylase gene, including all the mutations previously reported. PCR-SSCP analysis in eight patients showed at least one polymorphic site in each case. We confirmed that the mobility shifts in SSCP in an affected kindred were transmitted as a Mendelian trait. As these results indicated that PCR-SSCP profiles could be used for DNA-based diagnosis, we attempted to use this technique for prenatal diagnosis. DNA was obtained by chorionic villus sampling of a fetus and PCR-SSCP profiles were analysed in the PCR-amplified fragments in which the mobility shifts had been observed in the SSCP of the proband. We concluded that the fetus was a carrier. Direct nucleotide sequencing and allele-specific oligonucleotide hybridization confirmed that the fetus was heterozygous. At birth, the infant showed no signs of virilization or of abnormal endocrine findings on laboratory study. The results suggest that this new application of PCR-SSCP has advantages over conventional RFLP analysis and is useful in making a prenatal diagnosis of steroid 21-hydroxylase deficiency both rapidly and accurately.