Seventy-one patients with systemic sclerosis (SS) were typed for twenty-seven HLA alleles of the A and B loci, and the findings were related to both the extent of visceral disease and tests of cellular immune competence in a subgroup of fifty-two of these patients. Nineteen pa;ients with widespread visceral involvement and more rapidly progressive disease had an increased frequency of HLA-B8 (relative risk = 4.14; P less than 0.05) when compared to thirty-three less severely affected patients and 3000 controls. Patients with severe and progressive disease also had defective cell-mediated immunity with reductions in both the numbers of circulating thymus-dependent (T) lymphocytes and in the lymphocyte transformation response to phytohaemagglutinin. These findings suggest that a genetic factor, such as an abnormal immune response gene, may be involved in the progression of the disease.