SummaryThe conventional method of lung scanning detects defects of pulmonary artery perfusion. False positive results occur because regions of hypoventilation, such as are present in obstructive airways disease, also cause defects of perfusion. The converse is not true, however, as defects of perfusion continue to be ventilated. Thus in pulmonary embolism ventilation-perfusion discrepancy (normal ventilation and impaired perfusion) occursWe have assessed the clinical value of this discrepancy. Out of 18 patients with ventilation-perfusion discrepancy 14 had a final diagnosis of pulmonary emboli, whereas in none of the 34 patients without the discrepancy was this final diagnosis made. We conclude that combined ventilationperfusion lung scanning distinguishes pulmonary emboli from other lung conditions such as asthma and bronchitis which cause impaired pulmonary perfusion. The false positive rate was only 4% overall and 7-7% in patients with perfusion defects. IntroductionLung perfusion scanning is of value in the diagnosis of pulmonary embolism and is a simple and safe screening proced-

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