We identified patients with PE using the Pennsylvania Health Care Cost Containment Council database,14 which contains information on demographics, source of admission, admission and discharge dates, inpatient mortality data, and International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) discharge diagnosis and procedure codes for patients admitted to all nongovernmental acute care hospitals in Pennsylvania. Our study included inpatients 18 years or older who had a primary discharge diagnosis of acute PE between January 1, 2000, and November 30, 2002, based on the following ICD-9-CM codes: 415.1, 415.11, 415.19, and 673.20 through 673.24. To ensure that we identified the most severely ill patients with PE as the primary reason for hospitalization, we also included inpatients with a secondary ICD-9-CM code for PE and 1 of the following primary codes that may represent complications or treatments of PE: respiratory failure (ICD-9-CM code 518.81), cardiogenic shock (ICD-9-CM code 785.51), cardiac arrest (ICD-9-CM code 427.5), secondary pulmonary hypertension (ICD-9-CM code 416.8), syncope (ICD-9-CM code 780.2), thrombolysis (ICD-9-CM code 99.10), and intubation or mechanical ventilation (ICD-9-CM codes 96.04, 96.05, and 96.70-96.72). Because patients with recurrent PE may have a higher mortality than patients with a single episode,15- 16 we included all episodes of PE for the same patient within the study period to avoid potential selection bias. We did not include patients who had only a secondary ICD-9-CM code for PE or who were transferred from another health care facility, because such patients are more likely to have PE as a complication of hospitalization (eg, after surgery). Because outpatient treatment for PE was not considered usual care between 2000 and 2002, it is likely that we captured most patients having a primary diagnosis of PE in Pennsylvania during this period. This study was approved by the institutional review board of the University of Pittsburgh, Pittsburgh, Pa.