Summary. Background: Genetic testing for factor (F)V Leiden is widely performed in an effort to prevent thrombosis-related morbidity. The implications of a positive test for patients' health perception and the extent of patients' understanding of results are not known. Objectives: This study examined patient experience of genetic testing for FV Leiden. Patients and methods: The study was a cross-sectional, mailed survey of 110 patients who tested positive for the FV Leiden gene mutation at an academic medical center between 1995 and 2001. Patient knowledge about FV Leiden, satisfaction with available information, and psychosocial reactions to testing were assessed and the influence of demographic and clinical characteristics on outcome measured. Results: The magnitude of thrombosis risk associated with FV Leiden was incorrectly estimated by 79% of participants. Many patients (64%) stated that they had not been given much information about FV Leiden and 68% still had many questions. Most patients (53%) felt that their healthcare providers do not understand FV Leiden. Patients who had been seen by a hematologist or in a specialized thrombosis clinic were more knowledgeable and had less information need. Most patients (88%) were glad to know genetic test results, despite negative psychosocial implications such as increased worry (43%). Conclusions: Knowledge of genetic status increases awareness of thrombosis risk among patients, but magnitude of risk is often overestimated. Affected individuals indicate that there is a lack of available information about FV Leiden and that additional educational resources are needed.