The genetic structure of Pavia province is studied separately with HLA class I and II alleles. The analysis reveals that HLA class I genes reflect the same geographic barriers to migration found also with surnames, while class II gene distribution does not seem to be influenced by the presence of these barriers. The independence of HLA class II allelic distribution from the genetic structure of the area reinforces the hypothesis that environmental factors, rather than genetic drift, influence the frequency of these alleles. An example may be the significant correlation found between the distribution of the DR3-DQ2 haplotype and the prevalence of celiac disease in the province's districts.