Objectives: In older patients, the the D-dimer test for pulmonary embolism has reduced specificity and is therefore less useful. In this study a new, age dependent cut-off value for the test was devised and its usefulness with older patients assessed. Design: Retrospective multicentre cohort study. Setting: General and teaching hospitals in Belgium, France, the Netherlands, and Switzerland. Patients: 5132 consecutive patients with clinically suspected pulmonary embolism. Intervention: Development of a new D-dimer cut-off point in patients aged >50 years in a derivation set (data from two multicentre cohort studies), based on receiver operating characteristics (ROC) curves. This cut-off value was then validated with two independent validation datasets. Main outcome measures: The proportion of patients in the validation cohorts with a negative D-dimer test, the proportion in whom pulmonary embolism could be excluded, and the false negative rates. Results: The new D-dimer cut-off value was defined as (patient's agex10) μg/l in patients aged 70 years, ranging from 13% to 16% in the three datasets. The failure rates (all ages) were 0.2% (95% CI0%to 1.0%) in the derivation set and 0.6% (0.3% to 1.3%) and 0.3% (0.1% to 1.1%) in the two validation sets. Conclusions: The age adjusted D-dimer cut-off point, combined with clinical probability, greatly increased the proportion of older patients in whom pulmonary embolism could be safely excluded.