We have examined the distribution of HLA antigens in 70 multiple sclerosis (MS) patients divided in three groups defined according to clinical criteria: benign MS, severe MS, cerebellar MS. We have found a significant association between severe MS and HLA-DR2, and between benign MS of more than 15 years of evolution and HLA-DR3. We review previous work along the same line and conclude that the association of HLA antigens with "clinical subgroups of MS" could indicate a genetically based heterogeneity of the disease and offer help in establishing a prognosis.