Prenatal diagnosis and treatment of congenital adrenal hyperplasia due to steroid 21-hydroxylase (21-OH) deficiency has been proved to be effective. Screening for a panel of nine point mutations, deletions, and gene conversions allows the identification of most of the mutations, although 6-12 per cent of chromosomes remain uncharacterized. In the present study, microsatellite typing in the HLA region was performed in 23 21-OH deficiency families to determine the usefulness of these markers in the indirect identification of disease alleles. Two Génèthon markers (D6S273 and D6S439) in the HLA complex, class III and II regions in 5 and 3 , respectively to the CYP21 gene, were typed together with a microsatellite at intron 3 of the TAP1 gene also in 3 . The heterozygosity of these markers provided informativity in all but one family, in which only the father was informative. Direct genotyping of the chromosomes confirmed in each case the correct assignment of the disease alleles in the sibling. The indirect analysis of the 21-OH gene through D6S273, TAP1, and D6S439 microsatellites provides useful information in the molecular analysis of steroid 21-OH deficiency. 1997 by John Wiley & Sons, Ltd.