High levels of CXCL4 were observed in the circulation of patients with limited systemic sclerosis (SSc), late diffuse SSc, and early diffuse SSc in the identification cohort at Boston University (Panel A) and in 161 patients with corresponding subtypes of systemic sclerosis in two independent (Dutch and Swedish) replication cohorts, as compared with 129 age- and sex-matched healthy controls, 109 patients with systemic lupus erythematosus (SLE), 93 with ankylosing spondylitis (AS), and 93 with hepatic fibrosis (Panel B). The horizontal lines indicate means, and I bars standard deviations. Shown are the estimated times until the development of lung fibrosis (Panel C) and pulmonary arterial hypertension (PAH) (Panel D) within the first 24 months after the diagnosis of systemic sclerosis among patients with a high circulating CXCL4 level (≥10 ng per milliliter) and among those with a low CXCL4 level (<10 ng per milliliter).