In this case-control study, post-menopausal women with a first episode of objectively confirmed venous thromboembolism, and population-based controls were identified and recruited from the GHC enrollment files. Controls were frequency matched to the cases on age and calendar-year. Data collection included a review of ambulatory medical record and a telephone interview. The GHC computerized pharmacy database was used to assess exposure to hormone replacement therapy. A venous blood specimen was obtained from consenting subjects, processed into aliquots of white cells, plasma, and red cells, and stored at 70 degrees C prior to laboratory analysis. DNA was extracted from white cells, and molecular genotyping assays were conducted to assess carriership of prothrombotic mutations.