BACKGROUND: Anemia is frequent in patients with cancer, but there are concerns regarding treatment with erythropoiesis-stimulating agents. Blood transfusions are commonly used as an alternative, but with little data regarding outcomes. METHODS: In a retrospective cohort study, we investigated the associations between transfusions and venous thromboembolism, arterial thromboembolism, and mortality in hospitalized patients with cancer using the discharge database of the University HealthSystem Consortium, which included 504 208 hospitalizations of patients with cancer between 1995 and 2003 at 60 US medical centers. RESULTS: Of the patients included, 70 542 (14.0%) received at least 1 red blood cell (RBC) transfusion and 15 237 (3.0%) received at least 1 platelet transfusion. Of patients receiving RBC transfusions, 7.2% developed venous thromboembolism and 5.2% developed arterial thromboembolism, and this was significantly greater than the rates of 3.8% and 3.1%, respectively, for the remaining study population (P < .001). In multivariate analysis, RBC transfusion (odds ratio [OR], 1.60; 95% confidence interval [CI], 1.53-1.67) and platelet transfusion (1.20; 1.11-1.29) were independently associated with an increased risk of venous thromboembolism. Both RBC transfusion (OR, 1.53; 95% CI, 1.46-1.61) and platelet transfusion (1.55; 1.40-1.71) were also associated with arterial thromboembolism (P < .001 for each). Transfusions were also associated with an increased risk of in-hospital mortality (RBCs: OR, 1.34; 95% CI, 1.29-1.38; platelets: 2.40; 2.27-2.52; P < .001). CONCLUSIONS: Both RBC and platelet transfusions are associated with increased risks of venous and arterial thrombotic events and mortality in hospitalized patients with cancer. Further investigation is necessary to determine whether this relationship is causal.