The fibrin sleeve of venous catheters (VC) and parietal thrombi represent frequent and dangerous side-effects of central venous catheterization (CVC), due to the risk of embolism. Reduced levels of coagulation clotting factors inhibitors (such as Antithrombin III) are known to be associated with increased thrombogenic risk. The aim of this study was to evaluate the role of Antithrombin III (AT III) deficiency as a risk factor for thrombosis in cancer patients undergoing CVC. The study groups included patients with a reduced AT III activity (< 70%, 20 consecutive patients) and with normal AT III values (> 70%, 20 randomly selected patients), requiring a VC for chemotherapy and/or total parenteral nutrition. The study protocol included evaluation of Hb, PLTs, PT (INR), aPTT, Fibrinogen and AT III at days 0, 1, 3 and 8 after CVC and upon VC removal. Peripheral and pullout phlebographies were performed in all patients on catheter withdrawal. A quantitative scale was developed to evaluate both VC and parietal thrombus degree in each catheter-containing venous segment (subclavian, innominate, superior vena cava); the sum of the mean values was defined as overall thrombus. The average VC dwelling time was similar in both groups. There were no significant differences in Hb, PLTs, PT (INR), aPTT, Fibrinogen and in the remaining parameters of the study between the two groups. The group with AT III deficiency presented a higher degree of both parietal (p < 0.05) and overall thrombus (p < 0.02). Data showed a higher severity of CVC-related thrombosis in patients with AT III deficiency than in the control group. Further studies are needed to evaluate whether the therapeutically-induced normalization of AT III levels can reduce the thrombosis degree.