Evidence-based treatment guidelines rely critically on reliable estimates of the risks and benefits of treatment. Further, application of guidelines to specific populations requires information on the way these risks vary according to patient characteristics and time after disease onset and treatment initiation. New information from the population-based Worcester Venous Thromboembolism Study (1) provides compelling evidence that most previous estimates of the risks of bleeding after venous thromboembolism (VTE) substantially underestimate this risk in some patients. Specifically, the 6% risk of bleeding within 30 days after diagnosis in residents of the Worcester metropolitan area is substantially larger than bleeding risks seen among patients in randomized trials (2), and also more than twice the 90-day risk of bleeding after initiation of anticoagulant therapy in the RIETE registry (3). The size of the Worcester study excludes chance as an explanation for these differences, and the definition of bleeding outcomes precludes inflation of the estimate by inclusion of minor bleeds. Differences in study inclusion criteria are the most likely explanation for the heterogeneity in bleeding risks across studies. In particular, the Worcester study included many patients near death as well as complicated, actively bleeding patients who would not be included in randomized trials or registry studies drawing from anticoagulation clinics.