Characterization of the extent to which particular combinations of autoimmune diseases occur in excess of that expected by chance may offer new insights into possible common pathophysiological mechanisms. The goal of this study was to investigate the spectrum of polyautoimmunity (i.e. autoimmune diseases co-occurring within patients) and familial autoimmunity (i.e. diverse autoimmune diseases co-occurring within families) in patients with systemic sclerosis (SSc). A cross-sectional study of two convenience samples of patients with SSc, one in Canada and the other in Colombia, was performed. History of other autoimmune diseases in the SSc patients as well as a family history of autoimmunity was obtained. Of 719 patients, 273 (38%) had at least one other autoimmune disease. A total of 366 autoimmune diseases were reported, of which the most frequent were autoimmune thyroid disease (AITD, 38%), rheumatoid arthritis (RA, 21%), Sjögren's syndrome (18%), and primary biliary cirrhosis (4%). There were 260 (36%) patients with first-degree relatives with at least one autoimmune disease, of which the most frequent were RA (18%) and AITD (9%). Having at least one first-degree relative with autoimmune disease was a significant predictor of polyautoimmunity in SSc patients. No significant differences in polyautoimmunity or familial autoimmunity were noted between diffuse and limited subsets of disease. Our results indicate that polyautoimmunity is frequent in patients with SSc and autoimmune diseases cluster within families of these patients. Clinically different autoimmune phenotypes might share common susceptibility variants, which acting in epistatic pleiotropy may represent risk factors for autoimmunity. © 2008 Elsevier Ltd. All rights reserved.