End tidal carbon dioxide tension (PET,CO 2 ) is a surrogate for dead space ventilation which may be useful in the evaluation of pulmonary embolism (PE). We aimed to define the optimal PET,CO 2 level to exclude PE in patients evaluated for possible thromboembolism. 298 patients were enrolled over 6 months at a single academic centre. PET,CO 2 was measured within 24 h of contrast-enhanced helical computed tomography, lower extremity duplex or ventilation/perfusion scan. Performance characteristics were measured by comparing test results with clinical diagnosis of PE.PE was diagnosed in 39 (13%) patients. Mean PET,CO 2 in healthy volunteers did not differ from PET,CO 2 in patients without PE (36.3¡2.8 versus 35.5¡6.8 mmHg). PET,CO 2 in patients with PE was 30.5¡5.5 mmHg (p,0.001 versus patients without PE). A PET,CO 2 of o36 mmHg had optimal sensitivity and specificity (87.2 and 53.0%, respectively) with a negative predictive value of 96.6% (95% CI 92.3-98.5). This increased to 97.6% (95% CI 93.2-99.) when combined with Wells score ,4.A PET,CO 2 of o36 mmHg may reliably exclude PE. Accuracy is augmented by combination with Wells score. PET,CO 2 should be prospectively compared to D-dimer in accuracy and simplicity to exclude PE.