AimsWe studied the characteristics and clinical outcome related to diuretic response and the effects of serelaxin in patients hospitalized for acute heart failure (AHF). Methods and resultsRELAX-AHF was a double-blind, placebo-controlled trial, enrolling 1161 patients admitted to hospital for AHF who were randomized to 48 h i.v infusions of placebo or serelaxin (30 mu g/kg per day) within 16 h from presentation. Diuretic response was defined as weight kg/[(total i.v. dose)/40 mg] + [(total oral dose)/80 mg)] furosemide (or equivalent loop diuretic dose) up to day 5. Median diuretic response was -0.42 (-1.00, -0.14) kg/40 mg. A poor diuretic response was independently associated with Western-like region (Western Europe, North America, Israel, and Poland), lower diastolic blood pressure, the absence of oedema, higher blood urea nitrogen, and lower levels of aspartate aminotransferase and potassium (all P