['Despite the promising results of serelaxin as a new potential acute heart failure (HF) therapy, its clinical use preceded the understanding of the endogenous relaxin system in HF. We aimed to evaluate relaxin circulating levels in a population of acute HF and their association with clinical and echocardiographic parameters.', 'We included 117 patients from a registry of acute HF. Admission serum relaxin was measured using an enzyme-linked immunosorbent assay (ELISA) kit. Clinical, analytical, and echocardiographic parameters were compared between patients with relaxin levels above and below the median. Median age was 82 years [interquartile range (IQR) 72-87], 41% of the patients were male, and 63% had systolic dysfunction. Median serum relaxin was 31.4\u2009pg/mL (IQR 0.6-89.8). Patients with relaxin levels above the median had more peripheral oedema (89.8% vs. 68.4%, P = 0.004) and a significantly higher sodium retention score (mean 4.8\u2009±\u20091.5 vs. 3.6\u2009±\u20092.0, P < 0.001). These patients also had significantly higher systolic pulmonary arterial pressure [median 47.0 (IQR 36.0-61.0) vs. 34.5 (IQR 25.0-51.0) mmHg, P = 0.002], higher prevalence of right ventricular (RV) systolic dysfunction (28.1% vs. 10.3%, P = 0.02), RV dilation (31.0% vs. 5.3%, P < 0.001), and right atrial dilation (66.1% vs. 36.5%, P = 0.002), and less inferior vena cava diameter variability (40% vs. 60%, P = 0.009). No differences were noted regarding admission blood pressure, left chamber dimensions, or LV function.'] Are relaxin serum levels in acute heart failure associated with pulmonary hypertension and right heart overload?