An Ashkenazi Jewish family in which the mother and a son both have inclusion body myositis (IBM) is reported. The condition developed at an earlier age and was more rapidly progressive and less responsive to treatment in the son than in the mother or other IBM patients in our clinic. Genetic analysis showed that the mother carried alleles of the 8.1 MHC ancestral haplotype (AH; HLA-B8, DRB1*0301), which is found in 85% of IBM patients in Western Australia. The son did not inherit this haplotype, but carried alleles characteristic of the 52.1AH (HLA-B5, DRB1*1502) of paternal origin. The findings indicate that in this family either the 8.1AH or 52.1AH may carry susceptibility for IBM and that the 52.1AH is associated with a more severe and treatment-resistant form of the disease.