Blood clots that form in deep veins (deep venous thrombosis) sometimes travel to the lungs, a condition known as pulmonary embolism. Patients who survive a pulmonary embolus need treatment with blood thinners, since having another pulmonary embolus could prove fatal. Methods commonly used to detect pulmonary embolism often fail to provide a definitive diagnosis. By using information from the medical history, physical examination, electrocardiogram, and chest x-ray, a simple clinical model has recently been developed to evaluate the probability of pulmonary embolism before other tests are performed (“pretest probability”). A blood test—the d-dimer test—that detects breakdown products of blood clots is also now available. This test is used most effectively to rule out blood clots by the absence of breakdown products. Neither the simple clinical model nor the d-dimer test had been studied in detail in emergency departments.