purpose: To construct and test prospectively a bleeding risk index for estimating the probability of major bleeding in hospitalized patients starting long-term anticoagulant therapy. patients and methods: In an inception cohort of 617 patients starting long-term anticoagulant therapy in one hospital, data were gathered retrospectively and bleeding was classified using reliable explicit criteria. We constructed a bleeding risk index by identifying and weighting independent predictors of major bleeding using a multivariate proportional-hazards model. The bleeding risk index was tested in 394 other patients prospectively identified in a second hospital. The index was compared to physicians' predictions. results: Major bleeding developed before discharge in 61 of all 1,011 patients (6%). The bleeding risk index included four independent risk factors for major in-hospital bleeding: the number of specific comorbid conditions; heparin use in patients aged 60 years or older; maximal prothrombin or partial thromboplastin time 2.0 or more times control; liver dysfunction worsening during therapy. In the testing group, the index predicted major bleeding, which occurred in 3% of 235 low-risk patients, 16% of 96 middle-risk patients, and 19% of 63 high-risk patients (p <0.001). The bleeding risk index performed as well as physicians' predictions, and integration of the bleeding risk index with physicians' predictions led to a classification system that was more sensitive (p = 0.03) than physicians' predictions alone. In 86% of patients with a high risk of major bleeding, we identified specific ways of improving therapy, e.g., avoiding overanticoagulation and nonsteroidal anti-inflammatory agents. conclusion: The bleeding risk index provides valid estimates of the probability of major bleeding in hospitalized patients starting long-term anticoagulant therapy and complements physicians' predictions. The possibility that bleeding can be prevented in high-risk patients warrants prospective evaluation. © 1990.