The study concerns HLA typing of 261 patients with disseminated sclerosis. All patients were grouped for loci A and B. In addition, 94 were typed in HLA DR and 132 in mixed lymphocytic culture (DW2 only). Analysis of tissue group distribution among patients compared with a control population showed not only over-representation of the B7, DW2, A3 B7 and B7 DR2 types (as previously stressed by several workers), but also of A9 and of B8 DR3 and A1 B8 DR3 associations. Moreover, it would seem that there are two distinct clinical forms of disseminated sclerosis, each form being characterized by a specific antigenic HLA association. Side by side with the conventional and most common "remittent" form, which is partly responsive to treatment, has a relatively favourable course and is frequently associated with B7 and DW2/DR2, there appears to emerge around DR3, B8 DR3 and A1 B8 DR3 a rarer, "progressive" form of rapidly increasing severity and resistant to immuno-suppressants. This, however, is a mere hypothesis which needs to be confirmed by further studies on a larger scale.