Ten patients affected with 21-hydroxylase (21-OH) deficient late-onset adrenal hyperplasia were studied to determine the prevalence of a mutation at codon 281 of the functional 21-OH gene (CYP21B) that results in a valine to leucine amino acid shift. This mutation has been reported in eight unrelated late-onset adrenal hyperplasia patients of Ashkenazi Jewish descent possessing the human leukocyte antigen-B14,DR1 haplotype. Normally, there are two 21-OH genes; a pseudogene (CYP21A) and a functional gene (CYP21B). The aberrant codon 281 sequence is normally present only in CYP21A. In all of our late-onset adrenal hyperplasia patients, hybridization of an oligonucleotide probe specific for this mutation was demonstrated to CYP21A but not to CYP21B. The mutation at codon 281 of CYP21B does not appear to be a ubiquitous genetic marker for 21-OH deficient late-onset adrenal hyperplasia, suggesting that this disorder may demonstrate the same molecular heterogeneity as congenital adrenal hyperplasia.