Steroid 21 hydroxylase deficiency is a major cause of congenital adrenal hyperplasia (CAH) and is caused by genetic impairment of the gene (CYP21B). In the human genome, CYP21B is located within the MHC class III region on the short arm of chromosome 6. Most of the genes in this region are highly polymorphic and crowded. Also the CYP21B gene is accompanied by its pseudogene (CYP21A) and tandemly arranged with two genes of fourth component of complement. This highly complex gene cluster in this area may predispose genetic instability of CYP21, i.e. mutations. In this study, tried to investigate the frequency of duplication and deletion of CYP21 and patterns of the genetic alterations of these genes.We also compared the genetic alteration in normal subjects with those of the CAH patients. The results showed that 15% of the normal korean population have duplication or deletion of CYP21. There was one normal subject with heterozygous deletion of CYP21B. Of the 5 CAH patients examined, 2 were found to show abnormal patterns. One was a large-scale gene conversion and the other a gene conversion associated with deletion involving both CYP21B and C4 locus II gene. Through this study, we carne to the conclusion that the duplication or even deletion of CYP21 and C4 might be quite a common event in the Korean population and these rearrangements must be regarded as polymorphisms. It could contribute to a high incidencs of CAH by providing a genetic pool of instable CYP21.