1. All 44 patients with pulmonary embolism shown by angiography had abnormal scans. 2. None of the 21 patients with normal lung scans had emboli demonstrated angiographically. 3. There is a good correlation of emboli with focal lobar scan defects (85 per cent) and focal segmental scan defects (64 per cent). 4. Seventy-seven per cent of 53 patients judged as having a high probability of pulmonary embolism had angiographically demonstrated emboli. 5. The lung scan should be the initial screening procedure for pulmonary embolism and if the scan results are inconclusive (lack of focal lobar or segmental defects but abnormal, or if there is a corresponding roentgenographic abnormality), then angiography should be performed. 6. Pulmonary angiography should also be used to confirm the scan diagnosis prior to pulmonary embolectomy, inferior vena caval interruption or thrombolytic therapy.