The results of ventilation-perfusion (V-Q) imaging and pulmonary angiography were retrospectively analyzed in 146 patients with suspected pulmonary embolism (PE) to define the frequency of PE associated with various scintigraphic patients. When the radionuclide images demonstrated at least two moderate-sized or one large area of V-Q mismatch, the probability of PE was 92%. However, only one of three patients with a single moderate-size V-Q mismatch had PE, while small V-Q mismatches were not associated with PE in any of 19 patients. Matched V-Q abnormalities in lung regions that were radiographically normal were infrequently due to PE (4.8%). When a perfusion defect was substantially smaller than a corresponding radiographic abnormality, the frequency of PE was low (7.7%). Conversely, when a perfusion defect was substantially larger than the corresponding radiographic abnormality, there was a high probability of PE (87%). Matched perfusion and radiographic abnormalities indicated an intermediate probability of PE (27%). Patients with suspected pulmonary embolism may be classified into groups with low, intermediate, or high probability of pulmonary embolism on the basis of size and number of perfusion defects and a careful comparison of perfusion defects with ventilatory and radiographic findings.