To validate red cell distribution width (RDW) as an improvement in 30-day mortality risk stratification based on the Pulmonary Embolism Severity Index (PESI) in acute pulmonary embolism (PE). Prospective observational analysis of consecutive adult acute PE patients. Among 731 patients, 30-day mortality was 11.9%. With adjustment for the PESI score and number of covariates, higher RDW was associated with higher mortality (RDW continuous: OR 1.21, 95% CI 1.06–1.38; Bayesian OR 1.22, 1.07–1.40; RDW ‘high’ >14.5% in men >16.1% in women vs normal: OR 3.83, 1.98–7.46; Bayesian OR 3.98, 2.04–7.68. Crude mortality was 3.6% if PESI 86–105 (intermediate risk), but 1.2% if RDW normal and 7.1% if RDW high; 11.8% if PESI 106–125 (high risk), but 3.6% if RDW normal and 18.8% if RDW high. Adjusted probabilities showed higher mortality (ORs between 3.5–5.8) if RDW was high in any PESI risk subgroup. Crude mortality rates in two random-split subsets (n=365 and n=366) again showed the same patterns. On-admission RDW above the normal range improves 30-day mortality risk stratification based on PESI score in acute PE. Particularly, it corrects PESI-based intermediate-risk or high-risk allocation by reclassification into very low-risk (<3.5%) or very high-risk (>11.0%). •The Pulmonary Embolism Severity Index (PESI) is a well-validated tool for prediction of 30-day mortality in acute pulmonary embolism (PE) patients that still needs additional improvement in early mortality risk estimation among intermediate-risk and high-risk groups•There is a need for a simple, fast, cheap and widely available marker that could be used as a reliable add-on tool to PESI in an emergency room or any clinical or organisational setting•This study showed that red cell distribution width reliably reclassified a substantial number of PESI intermediate-risk and high-risk patients into low-risk or very high-risk categories, and that its simple dichotomised use could further improve decision-making in acute pulmonary embolism in the emergency room setting with limited resources