Multiple sclerosis has been shown to be associated with the presence of certain major histocompatibility (MHC) tissue antigens which are coded on chromosome 6. There are racial differences in the antigens associated with MS. The strength of the associations vary in different communities in Western Europe. We have investigated the association between MS and MCH antigens in Northern Ireland in a group of 104 patients. There is a particularly strong association between MS and HLA-DR2, 65.4% compared with 25.5% in 184 controls. A weaker association has been demonstrated with HLA-A3 (44.2% vs 26.5% in controls). There have been conflicting reports concerning an association of HLA-DW2 and HLA-DR2 with a rapidly progressive form of MS. Our data do not support that hypothesis.