Late-onset adrenal hyperplasia (LOAH) due to 21-hydroxylase (21-OH) deficiency is one of the most common autosomal recessive disorders. There appear to be two 21-OH genes, CYP21A (a pseudogene) and CYP21B (the functional gene), which lie in close proximity to the human leukocyte antigen (HLA) encoding region on the short arm of chromosome 6. While the CYP21A/CYP21B ratio is normally 1:1, ratio abnormalities are frequent in LOAH, suggesting gene deletion, duplication or conversion. The objective of this study was to determine whether an abnormal CYP21A/CYP21B ratio could predict carriers of LOAH, as determined by endocrine and HLA results. The probands appear to be compound heterozygotes carrying a 21-OH gene for LOAH and a deletion of the homologous gene. However, concordance between an abnormal 21-OH gene ratio and the inheritance of the LOAH gene does not appear to be complete, as demonstrated by this family study. Further studies of the feasibility of screening carriers for 21-OH deficiencies with the CYP21A/CYP21B ratio or other molecular probes must be performed.