We conducted a retrospective, population-based study to identify all Olmsted County residents with first-lifetime onset of deep vein thrombosis or pulmonary embolism from January 1, 1966, through December 31, 1990. A master list of potential Olmsted County residents with deep vein thrombosis, pulmonary embolism, pulmonary infarction, or similar diagnoses or who had any diagnostic test or procedure used in the diagnosis of deep vein thrombosis or pulmonary embolus, was constructed by searching the computerized indexes of medical diagnoses and surgical procedures, as well as all available databases for diagnostic tests, billing data, death certificates, and autopsy diagnoses for Olmsted County residents. The following data sources were included. First, the Medical Diagnostic Index was searched to identify all patients with diagnoses of (a) deep vein thrombosis, phlebitis, thrombophlebitis, phlegmasia alba dolens, phlegmasia cerulea dolens, and similar terms for venous thrombosis; (b) pulmonary embolism, pulmonary infarction, or similar diagnoses; and (c) varicose veins, varicose ulcer, stasis ulcer, venous insufficiency, postphlebitic syndrome, superficial vein thrombosis, venous stasis, and similar diagnoses; and (d) patients undergoing a radionuclide study for venous thromboembolism, including perfusion or ventilation-perfusion lung scans, radiolabeled fibrinogen leg scanning, or radionuclide venography. Second, the Surgical Procedure Index was searched to identify all patients who underwent a surgical procedure for deep vein thrombosis or pulmonary embolism, including vein ligation, inferior vena cava ligation or interruption, placement of an inferior vena cava filter or similar device, and venous or pulmonary thrombectomy. Third, multiple radiology databases of the Mayo Clinic were searched for patients who underwent venography, pulmonary angiography, duplex ultrasonography, computed tomography, or magnetic resonance imaging to identify any additional patients with diagnoses of deep vein thrombosis, pulmonary embolism, or pulmonary infarction. (During the study period all computed tomography and magnetic resonance imaging studies for Olmsted County residents were performed at the Mayo Clinic and their diagnoses were included in the radiology databases.) Fourth, the echocardiography computer database of the Mayo Clinic was searched to identify patients with diagnoses of pulmonary embolism, pulmonary hypertension, or similar diagnoses. Fifth, data from the Mayo Clinic vascular laboratory logbook were entered into a computer database for this study, and all patients undergoing impedance plethysmography, Doppler ultrasonography, or duplex ultrasonography for venous disease were identified. Finally, computerized billing databases of the Mayo Clinic were searched to identify all patients with charges for noninvasive vascular laboratory tests, including impedance plethysmography and Doppler ultrasonography. The final master list of potential cases contained 9046 individuals whose complete (inpatient and outpatient) medical records from all providers of health care in Olmsted County were reviewed for the study.