The aim of this study was to examine the value of measurement of the interleukin-1 receptor family member ST2 in patients with dyspnea. Concentrations of ST2 have been reported to be elevated in patients with heart failure (HF). Five hundred ninety-three dyspneic patients with and without acute destabilized HF presenting to an urban emergency department were evaluated with measurements of ST2 concentrations. Independent predictors of death at 1 year were identified. Concentrations of ST2 were higher among those with acute HF compared with those without (0.50 vs. 0.15 ng/ml; p < 0.001), although amino-terminal pro-brain natriuretic peptide (NT-proBNP) was superior to ST2 for diagnosis of acute HF. Median concentrations of ST2 at presentation to the emergency department were higher among decedents than survivors at 1 year (1.08 vs. 0.18 ng/ml; p < 0.001), and in multivariable analyses, an ST2 concentration > or =0.20 ng/ml strongly predicted death at 1 year in dyspneic patients as a whole (HR = 5.6, 95% confidence interval [CI] 2.2 to 14.2; p < 0.001) as well as those with acute HF (hazard ratio [HR] = 9.3, 95% CI 1.3 to 17.8; p = 0.03). This risk associated with an elevated ST2 in dyspneic patients with and without HF appeared early and was sustained at 1 year after presentation (log-rank p value <0.001). A multi-marker approach with both ST2 and NT-proBNP levels identified subjects with the highest risk for death. Among dyspneic patients with and without acute HF, ST2 concentrations are strongly predictive of mortality at 1 year and might be useful for prognostication when used alone or together with NT-proBNP.