Objectives: To quantify the prognostic utility of QRS and QTc interval prolongation in patients presenting with acute destabilised heart failure (ADHF) to the emergency department (ED). Design: Prospective cohort study among patients enrolled in the B-Type Natriuretic Peptide for Acute Shortness of Breath Evaluation (BASEL) study. QRS and QT intervals were measured in 173 consecutive patients with ADHF. QT interval was corrected using the Bazett formula. The primary end point was all-cause mortality during the 720-day follow-up. Results: QRS interval was prolonged (⩾120 ms) in 27% of patients, and QTc interval was prolonged (⩾440 ms) in 72% of patients. Baseline demographic and clinical characteristics were comparable in patients with normal and prolonged QRS or QTc intervals. A total of 78 patients died during follow-up. Interestingly, the 720-day mortality was similar in patients with prolonged and normal QTc (44% vs 42%, p = 0.546), but was significantly higher in patients with prolonged QRS interval than in those with normal QRS (59% vs 37%, p = 0.004). In Cox proportional hazards analysis, prolonged QRS interval was associated with a nearly twofold increase in mortality (HR 1.94, 95% CI 1.22 to 3.07; p = 0.005). This association persisted after adjustment for variables routinely available in the ED. Conclusions: Prolonged QRS interval, but not prolonged QTc interval, is associated with increased long-term mortality in patients with ADHF.