The relation between clinical and serological findings was studied in 75 patients with definite progressive systemic sclerosis (PSS). Antinuclear antibodies were detected by indirect immunofluorescence in 95% of the patients by means of HEp-2 cells. The centromere pattern was observed in 50% of the patients with calcinosis, Raynaud's phenomenon, oesophageal dysmotility, sclerodactyly, and telangiectasia (CREST) and only 7% of the group with diffuse disease. Precipitating antibodies to nuclear and cytoplasmic antigens were detected by immunodiffusion in 53% of the patients and identified as anti Scl-70 in 16 (21%). We confirmed that autoantibodies are very frequent in PSS but varied in specificity. A combination of immunodiffusion and indirect immunofluorescence was useful in detecting antibodies characteristic of PSS (i.e., anti-Scl-70 and anticentromere) in 51% of these patients. Anticentromere antibody has a high specificity for CREST and identifies patients with less severe disease (i.e., more prolonged course and less involvement of internal organs). Anti-Scl-70 was associated with a high frequency of lung involvement.