Aim: Patients in medical intensive care units (MICU) are at increased risk for venous thromboembolism (VTE) and prophylaxis is recommended. However, the range and frequency of VTE prophylaxis administered to MICU patients in Korea are not well defined. Methods: Patients over 40 years of age and expected MICU stay of more than 48 hours were eligible for this observational cohort study of 23 MICUs. Patients on anticoagulation before admission or those requiring anticoagulation for reasons other than VTE were excluded. Patients with contraindication to mechanical prophylaxis or those with recent operation were also excluded. This study was sponsored by sanofi-aventis Korea Co., Ltd. Results: Among 830 patients, 553 were male and median age was 71 years. The average body mass index was 21.6 kg/m2. VTE prophylaxis was given to total 560 (67.5%) patients. Among these patients, 323 received chemoprophylaxis, 318 received mechanical prophylaxis and 81 received both forms of prophylaxis. A total of 441 patients had 1 or more risk factors for VTE and the most common risk factor was old age (＞65 years). Patients without risk factors for VTE before admission to MICU were 5 times more likely to not receive chemical prophylaxis (p＜0.001). Patients who were not given any prophylaxis were significantly younger (p＜0.001) compared with those on VTE prophylaxis. Conclusion: In this observational cohort study of 23 MICUs, 67.5% of patients received VTE prophylaxis. Further studies are needed to determine the incidence of VTE and the range of prophylaxis in Korean critically-ill patients.