Background: Current evidence points to a state of hypercoagulability (consequence of hyperinflammation) as an important pathogenic mechanism that contributes to the increase in mortality in cases of COVID-19. The aim of the present study was to investigate the influence of deep-vein thrombosis on mortality patient’s infected with SARS-CoV-2. Method: A clinical trial was conducted involving 200 consecutive patients with COVID-19—100 patients who were positive for deep-vein thrombosis (venous Doppler ultrasound) and 100 who were negative for deep-vein thrombosis at a public hospital. Results: The mortality rate was 67% in the group positive for DVT and 31% in the group negative for DVT. Conclusion: Deep-vein thrombosis is associated with an increase in mortality in patients with COVID-19 and failures can occur with conventional prophylaxis for deep-vein thrombosis.