Venous thromboembolism (VTE) is increasingly diagnosed among individuals with hematologic malignancies. However, the risk of VTE among patients undergoing hematopoietic stem cell transplantation (HSCT) is unclear. We examined the incidence and risk factors for VTE and bleeding among 1514 patients undergoing in-patient HSCT. No protocolized VTE prophylaxis was used. By HSCT day 180, 75 symptomatic VTE occurred in 70 patients (4.6%; 95% confidence interval [CI], 3.6%-5.8%). Fifty-five (3.6%) were catheter-associated, 11 (0.7%) were non-catheter-associated deep venous thromboses, and 9 (0.6%) were pulmonary emboli. Thirty-four percent of VTE occurred at a platelet count less than 50 x10(9)/L; 13% occurred at a platelet count less than 20 x10(9)/L. In multivariate analysis, VTE was associated with prior VTE (odds ratio [OR], 2.9; 95% CI, 1.3-6.6) and with graft-versus-host disease (GVHD; OR, 2.4; 95% CI, 1.4-4.0). Clinically significant bleeding occurred in 230 patients (15.2%; 95% CI, 13.4%-17.1%); 55 patients (3.6%; 95% CI, 2.7%-4.7%) had fatal bleeding. Bleeding was associated with anticoagulation (OR, 3.1; 95% CI, 1.8-5.5), GVHD (OR, 2.4; 95% CI, 1.8-3.3), and veno-occlusive disease (OR, 2.2; 95% CI, 1.4-3.6). In HSCT patients, VTE is primarily catheter-related and 3-fold less common than clinically significant bleeding. These findings warrant consideration when selecting VTE prophylaxis in HSCT patients.