['Stasis of the blood has been postulated as a major cause of venous thrombosis. However, little is known about the effect of stimulating the blood flow in order to prevent venous thrombosis through, for example, sports activities.', 'In a large population-based case-control study (MEGA study), we studied whether participating in sports activities on a regular basis was associated with venous thrombosis risk.', 'Consecutive patients with a first venous thrombosis of the leg or a pulmonary embolism, and control subjects, consisting of partners of the patients and randomly selected control subjects from the general population, were asked to participate. Sports activities and other risk factors for venous thrombosis were reported in a standardized mailed questionnaire. Participants with malignancy were excluded.', 'Out of 3608 patients, 1136 (31.5%), and of our 4252 control subjects 1686 (39.7%), participated in sports activities. Participating in sports activities reduced the risk of venous thrombosis compared with not participating in sports activities [odds ratio (OR) 0.64; 95% CI 0.58-0.71]. Risk reductions were similar after adjustment for sex, age and body mass index (OR(adj) 0.71; 95% CI 0.64-0.78) and when the analysis was restricted to healthy individuals (OR(adj) 0.67; 95% CI 0.58-0.78). No differences in risk were found for various frequencies, intensities and types of sport.'] Do regular sports activities decrease the risk of venous thrombosis?