We determined the prevalence of 24 antigens controlled by the HLA-A and B loci in twenty patients with juvenile diabetes mellitus (JDM), in twenty patients with coeliac disease (CD), and in eight patients with both of these diseases. The prevalence of HLA-B8 was increased in JDM chi2 = 12.52, p = 0.00040) and in CD (chi2 = 26.47, p less than 0.000001) as compared to 900 controls. There was only a modest increase of Bw15 in JDM (chi2 = 8.86, p = 0.0029) and in patients with both diseases (chi2 = 2.72). The observed prevalence of phenotype HLA-B8, Bw15 was enhanced in JDM (chi2 = 16.03, p = 0;000063) and in patients with both JDM and CD (chi2 = 24.48, p = 0.00000074) as compared with controls. In the latter group the observed value was 2.2 fold to that expected. In family studies the children having both B8 and Bw15 were more disposed to develop diabetes than siblings with only one of these antigens. In conclusion, the inherited susceptibility to develop juvenile diabetes is markedly associated with HLA-B8 and slightly with HLA-Bw15, and that of coeliac disease with HLA-B8 in Finnish paediatric patients. The presence of both B8 and Bw15 simultaneously increases the susceptibility to have both JDM and CD.