Five hundred unrelated Danes, including 202 healthy individuals, 35 cadaveric kidney donors, and 263 patients in terminal uraemia were typed for the HLA-DR antigens DR1-w8. The HLA-DR gene frequencies of healthy Danes are in good agreement with frequencies estimated during The Eighth International Histocompatibility Workshop of the Scandinavian population, but differ to some extent from other groups of European Caucasians. This indicates geographical variation in the distribution of DR genes within Europe. Very close associations were found between the HLA-DR antigens and their corresponding HLA-D specificities, except for Dw4 and Dw6, which were included in DR4 and DRw6, respectively. Linkage disequilibrium was calculated between alleles of two loci (HLA-A, -DR; -B, -DR and -C, -DR alleles was assessed for multiple loci by using haplotype data from 32 HLA-A, -B, -C, -D, -DR, and Bf typed Danish families. The haplotype data showed that HLA-DR4 is strongly associated with HLA-B15 only in haplotypes together with HLA-Dw4. This indicates genetic heterogeneity of HLA-DR4 which, however, does not appear serologically in this study.