In 102 patients suspected of pulmonary embolism, we have assessed the ability of a radionuclide (emission) venogram to complement the pulmonary perfusion scintigram in establishing a diagnosis. The efficacy was compared using decision analysis and Bayes's theorem. Two criteria for a positive test were compared: Criterion 1--the test is positive if the lung scan a) indicates a high probability of pulmonary embolus, or b) is abnormal but indeterminate for pulmonary embolus; Criterion 2--the test is positive a) if the lung scan indicates a high probability of pulmonary embolus, or b) if the emission venogram is positive in a patient with a lung scan considered abnormal but indeterminate for pulmonary embolus. The use of Criterion 2 decreased the sensitivity from 100% to 95% and increased the specificity from 74% to 93%. We conclude that a simultaneous emission venogram assists in improving the specificity, accuracy, and the predictive value of a standard pulmonary perfusion study in the diagnosis of pulmonary embolus.