Introduction: Genotype-phenotype discordance occurs occasionally in congenital adrenal hyperplasia (CAH). Its causes are largely unknown. We describe a case of monochorionic, diamn iotic twins with discordant clinical presentations of CAH, and show evidence for this being due to mosaicism resultin g from a postzygotic full gene deletion of CYP21A2 prior to twinning. Case description: A 7-day-old 36-week gestation female infant (Twin A) presented to the emergency department with elevated 17-hydroxyprogesterone (17-OHP). Her identical twin (Twin B) had normal 17-OHP on newborn screening. Both twins showed signs of virilization, more pronounced in Twi n B. Molecular genetic testing of both twins and their parents showed a WT paternally-inherited CYP21A2 and a maternally-inherited copy containing the c.293-13C>G mutation. Both twins were also found to have a 5'-CYP21A1P/CYP21A2-3' hybrid (product of the common 30-kb deletion), derived from the deletion of the paternally-in herited CYP21A2. Neither mother nor father carried the deletion. Conclusions: The genetic findings are consistent with mosaicism for two CYP21A2 cell lines in both twins. The first cell line is expected, based on parental results, while the sec ond line is due to a postzygotic full gene deletion of the paternally-inherited WT CYP21A2. The resultant genotype, compound heterozygosity for c.293-13C>G and a CYP21A2 full gene deletion, is consistent with a salt-wasting CAH pheno type. Differential distribution of the second cell line between the twins is most likely the cause for their discrepant phenotypes. We believe this is the first report of somatic CYP21A2 mosaicism, and represents a novel cause for discrepant CAH phe notypes in monozygotic twins. [ABSTRACT FROM AUTHOR]