COVID-19 predisposes to venous thromboembolism and there are multiple data regarding high incidence of venous thrombosis in critical COVID-19 patients, however reports on this complication in less severe patients are not widely available. The aim of this study was to investigate the incidence of deep-vein thrombosis (DVT) in patients with moderate to severe COVID-19 and to assess the prevalence of DVT with lung computerized tomography (lung CT) exams, clinical information and lab data. This study examined 75 consecutive patients with moderate to severe COVID-19, with specific exclusions. METHODS Almost all patients (pts) admitted to our hospital in the first half of May underwent comprehensive vein ultrasonography. 75 pts (aged 27-92 y, median - 63 y, 36 males and 39 females) with moderate to severe COVID-19 were included in our study. RESULTS Spontaneous echo contrast (decreased blood velocity and blood stasis) was detected in common femoral veins in 53 pts (70.7%). DVT was found in 15 pts (20%). The vast majority of those with DVT (13 pts, 86.7%) had thrombi only in calf veins and ileofemoral thrombosis was detected in 2 pts with DVT (13.3%). There was no significant observed difference between DVT and non-DVT patients with respect to age, underlying diseases, lung CT scores and SpaO2 at admission. There was also no significant observed difference between DVT and non-DVT patients with respect to both "time from symptoms onset to admission" and with respect to the majority of lab data. However, a significant difference was observed in D-dimer level (1.87 +/- 1.62 vs 0.51 +/- 0,4 mcg/mL p<0.0001) and C-reactive protein (116.9 +/- 83,6 and 65.1 +/- 64.98 mg/L, p = 0.014) for patients with DVT and patients without DVT respectably (Receiver operating characteristics (ROC) curve analysis revealed that the level of D-dimer >/= 0.69 mcg/mL is the predictor of DVT with a sensitivity of 76.9%, a specificity of 77.6%, p < 0.001 (AUC area under curve = 0.7944). Logistic regression confirmed that D-dimer is an independent predictor of DVT and patients with D-dimer >/= 0.69 mcg/mL have odds ratio (OR) of developing DVT = 5.1 (confidence interval [CI] 1.9 - 13.5)). CONCLUSION Patients with moderate to severe COVID-19 show high incidence of DVT, indicating that moderate to severe COVID-19 patients may require an early administration of anticoagulation therapy as part of their treatment. Such therapy may be continued after hospital discharge. Based on these findings, these patients may also require a follow-up with vein ultrasonography after recovery to rule out DVT.