The clinical importance of asymptomatic proximal and distal deep vein thrombosis (DVT) remains uncertain and controversial. The aim of this retrospective, post-hoc analysis was to examine mortality and risk factors for development of proximal DVT in hospitalized patients with acute medical illness who were recruited into a randomized, prospective clinical trial of thromboprophylaxis with dalteparin (PREVENT). We analyzed 1738 patients who had not sustained a symptomatic venous thromboembolic event by Day 21 and who had a complete compression ultrasound of the proximal and distal leg veins on Day 21. We examined the 90-day mortality rates in patients with asymptomatic proximal DVT (Group 1, N = 80), asymptomatic distal DVT (Group 11, N = 118) or no DVT (Group 111, N = 1540). The 90-day mortality rates were 13.75%, 3.39%, and 1.92% for Groups I-III, respectively. The difference in mortality between Group I and Group III was significant (hazard ratio 7.63,95% Cl = 3.8-15.3;p < 0.0001), whereas the difference between Groups 11 and III did not reach significance (hazard ratio 1.36,95% Cl = 0.41-4.45). The association of asymptomatic proximal DVT with increased mortality remained highly significant after adjusting for differences in baseline demographics and clinical variables. Risk factors significantly associated with the development of proximal DVT included advanced age (p = 0.0005), prior DVT (p = 0.001), and varicose veins (p = 0.04). In conclusion, the high mortality rate in patients with asymptomatic proximal DVT underscores its clinical relevance and supports targeting of asymptomatic proximal DVT as an appropriate endpoint in clinical trials of thromboprophylaxis.