Of 397 systemic sclerosis (scleroderma) patients from the University of Pittsburgh, who had serum determinations of both anti–Scl-70 and anticentromere antibody (ACA), 26% had anti-Scl-70 and 22% had ACA. No patient had both autoantibodies. Weak associations with HLA–DR5 and HLA-DR1 were detected with anti-Scl-70 and ACA, respectively. ACA was found almost exclusively (96%) in patients with limited cutaneous scleroderma (the CREST syndrome variant), but the majority (57%) of patients with limited scleroderma did not have this antibody. Among patients with limited scleroderma, those with ACA more often had calcinosis and telangiectasias and less often had pulmonary interstitial fibrosis and restrictive lung disease. However, the frequency of pulmonary hypertension and the survival rates were similar in the ACA + and ACA- limited scleroderma patients. Two-thirds of patients with anti-Scl-70 had diffuse scleroderma, but only 33% of all diffuse scleroderma patients had this antibody. Within the subset of diffuse scleroderma, anti-Scl-70 was associated with peripheral vascular disease (digital pitting scars) and pulmonary interstitial fibrosis, but was not predictive of cardiac or renal involvement or survival. ACA and anti–Scl-70 are useful in diagnosing and classifying systemic sclerosis variants and in predicting the natural course of the disease. Their mutually exclusive occurrence suggests either 2 separate clinical entities or important host factors determining their production.