AbstractDirect deciphering of past genes may reflect real characteristics of forebears, even of whole ancestral populations. This is obviously one of the most powerful and direct methods to follow evolutionary changes of the species. We attempted to apply ancient DNA (aDNA) technology to analyse a polymorphism at codon 129 of PRNP which probably plays a role in susceptibility to a variant Creutzfeldt - Jakob (vCJD) disease. As previously suggested, 129 Val-Val and heterozygous individuals are nearly completely protected from vCJD, in contrast to 129 Met-Met homozygous ones. We examined the frequency of the alleles encoding methionine and valine at codon 129 in DNA isolated from 100 skeletal remains of individuals who lived between 10th and 13th century. Our results confirmed significant alteration in previously studied alleles frequency between the populations of medieval Polish Lands and contemporaries. The calculated frequency of the alleles in medieval Poland (51% as compared to contemporary 65% for 129Met, and appropriately 49% vs. 35% for 129Val) implies a selection process that shaped 129 Met-Val distribution profiles in the Middle Ages. We suggest that the study of the genetic relationship between past and present-day populations could be a useful tool to follow allelic composition of particular genes (here: of the PRNP) over a span of time which may contribute to the understanding of evolutionary and selective mechanisms including epidemiological cases.