We have designed a rapid and convenient strategy to determine nine of the most common mutations in the 21-hydroxylase gene (CYP21). The frequency of the mutations was investigated in 34 Japanese patients affected with congenital adrenal hyperplasia (CAH) caused by 21-hydroxylase deficiency. We characterized 82% of the CAH chromosomes. The most frequent mutations were a C/A to G substitution in intron 2 in the salt-wasting form of the disease and an I172N in the simple virilizing form. Three de novo mutations were found. Two homozygous mutations (S268T and N493S) were detected by direct sequencing of all exons of CYP21 in two siblings, who had a normal genotype at all positions screened. We successfully applied these methods for prenatal diagnosis in one family. These procedures proved to be sensitive and rapid for the detection of the most common known mutations in the CYP21 gene and may be useful for genetic screening.