Objective. To investigate the contributions of the major histocompatibility complex (MHC) and C4 alleles to systemic sclerosis (SSc), and to pulmonary fibrosis and autoantibody expression in SSc, by analysis at the DNA level.Methods. One hundred fifteen patients with SSc were tested serologically for alleles of the class I MHC loci, and were tested for class I1 alleles (DRB, DQA, and DPB) by a combination of restriction fragment length polymorphism (RFLP) analysis and oligonucleotide probes with polymerase chain reaction amplification. C4 was studied by protein phenotyping and RFLP analysis in 80 patients. Correlations were made between disease status, pulmonary fibrosis, and expression of anticentromere antibodies (ACA) and antScl-70.Results. The C4A-null phenotype was found to provide the strongest disease association factor of the Conclusion. Of all the potential markers of disease susceptibility analyzed, the C4A locus was the strongest. C4AQO and DQAZ are independent susceptibility factors for SSc. The development of pulmonary fibrosis in SSc patients can be predicted using combined MHC and autoantibody analysis. The MHC alleles associated with the expression of disease-specific autoantibodies are not markers for disease susceptibility. Scleroderma (systemic sclerosis; SSc) and related disorders comprise a spectrum of acquired diseases characterized by fibrosis and vasculopathy . The spectrum ranges from localized scleroderma (morphea), where the emphasis is on dermal fibrotic disease, to diffuse cutaneous SSc (dcSSc) and limited cutaneous SSc (1cSSc) ( l ) , in which, to varying degrees, both the fibrotic and vascular elements are present and internal organ involvement is common.SSc is not considered to be an inherited disease, but there is a genetic association with the major histocompatibility complex (MHC) (see below). The human MHC comprises a discontinuous set of loci determining cell surface glycoproteins, which are spe-