In a population of patients experiencing thrombocytopenia while treated with heparin, bleeding and thromboses are well-appreciated complications, but their relative contributions to mortality have been less well described. In this population, the aims of this study were (1) to identify the independent predictors of bleeding and (2) to compare the incidence and the strength of association of bleeding and of new thromboses to in-hospital mortality. The independent predictors of bleeding and in-hospital mortality were identified using multivariate logistic regression models on the 1,478 patients who developed thrombocytopenia after their enrollment in the Complications After Thrombocytopenia Caused by Heparin (CATCH) study. The independent predictors of bleeding were chronic hematologic disorders, intra-aortic balloon pump, congestive heart failure, and platelet count nadir <120 x 10(9)/L. Although bleeding (n = 141 [10%]) and thromboembolic complications (n = 135 [9%]) were equally prevalent, the former was less strongly associated than the latter with in-hospital mortality (odds ratio 1.75, 95% confidence interval 1.01 to 3.03, and odds ratio 2.77, 95% confidence interval 1.67 to 4.61, respectively). In conclusion, medical management should be directed mainly at the prevention of thromboembolic complications, while additionally considering the risk for bleeding.