Patients admitted to medical intensive care unit (MICU) are at increased risk for venous thromboembolism (VTE); and prophylaxis is recommended. However, the actual range and frequency of VTE prophylaxis administered to MICU patients are not well defined. Patients over 40 yr of age and expected MICU stay of more than 48 hr were eligible for this observational cohort study of 23 MICUs in Korea. Patients already on anticoagulation therapy or those requiring anticoagulation for reasons other than VTE were excluded. Among 830 patients, VTE prophylaxis was given to 560 (67.5%) patients. Among 560 patients, 323 (38.9%) received pharmacoprophylaxis, 318 (38.4%) received mechanical prophylaxis and 81 (9.8%) received both forms of prophylaxis. About 74% of patients in the pharmacoprophylaxis group received low molecular weight heparin and 53% of the patients in the mechanical prophylaxis group used intermittent pneumatic compression. Most of the patients (90%) had more than one risk factor for VTE and the most common risk factor was old age, followed by heart and respiratory failure. In this observational cohort study of 23 MICUs in Korea, 67.5% of patients received thromboprophylaxis. Further studies are needed to clarify the role and efficacy of VTE prophylaxis in Korean critically ill patients.