Most central venous catheter (CVC)-related deep vein thromboses (DVT) are asymptomatic and their incidence and clinical relevance are still under debate. Data on CVC-related fibrin sheaths are scarce. We investigated the incidence of asymptomatic DVT and fibrin sheaths in cancer patients with long-term CVC implantation who underwent Doppler ultrasound surveillance at 1, 6, and 12 months after implantation. Effects of low-weight molecular heparin (LWMH) therapy on DVT and fibrin sheaths were also analyzed.