BACKGROUND: The Pulmonary Embolism Rule-out Criteria (PERC) rule is a clinical diagnostic rule designed to exclude pulmonary embolism (PE) without further testing. We sought to externally validate the diagnostic performance of the PERC rule alone and combined with clinical probability assessment based on the revised Geneva score. METHODS: The PERC rule was applied retrospectively to consecutive patients who presented with a clinical suspicion of PE to six emergency departments, and who were enrolled in a randomized trial of PE diagnosis. Patients who met all eight PERC criteria [PERC((-))] were considered to be at a very low risk for PE. We calculated the prevalence of PE among PERC((-)) patients according to their clinical pretest probability of PE. We estimated the negative likelihood ratio of the PERC rule to predict PE. RESULTS: Among 1675 patients, the prevalence of PE was 21.3%. Overall, 13.2% of patients were PERC((-)). The prevalence of PE was 5.4% [95% confidence interval (CI): 3.1-9.3%] among PERC((-)) patients overall and 6.4% (95% CI: 3.7-10.8%) among those PERC((-)) patients with a low clinical pretest probability of PE. The PERC rule had a negative likelihood ratio of 0.70 (95% CI: 0.67-0.73) for predicting PE overall, and 0.63 (95% CI: 0.38-1.06) in low-risk patients. CONCLUSIONS: Our results suggest that the PERC rule alone or even when combined with the revised Geneva score cannot safely identify very low risk patients in whom PE can be ruled out without additional testing, at least in populations with a relatively high prevalence of PE.