This review describes the presently available data on HLA typing in patients with multiple sclerosis (MS). Almost 2,000 MS patients and 12,000 normal controls have been studied with HLA typing in 15 different laboratories. The combined data show remarkably homogeneous results, and are as summarized: an increased frequency of HLA-Dw2 of B7, B8, Bw18, A3, and A1. A decreased frequency of A2, A28, B12, Bw15, Bw17, Bw22, and Bw40. Analysis of data seems to indicate that the HLA linked MS susceptibility gene is linked to HLA-Dw2, and the other deviating HLA antigen frequencies, except B8, may be explained as a secondary phenomenon, due to linkage disequilibrium. The presented data do not allow any conclusions as to whether the MS susceptibility gene operates in a dominant or recessive way. The important of HLA as a marker for disease and MS in particular is discussed, and preliminary data on HLA studies of MS patients of other ethnic groups, notably American blacks, seem to indicate that genetically determined ethnic differences, notably those of HLA, may partly be responsible for the unique geographic distribution of the disease.