The purpose of the current study was to evaluate the incidence and course of upper-extremity deep vein thrombosis (UEDVT) related to an implanted central venous port (CV-port) system in cancer patients. From July 2007 to July 2008, 92 consecutive patients who underwent implantation of a CV-port for chemotherapy for colorectal cancer were prospectively enrolled in the study. All patients were examined at prescribed intervals by ultrasonography (US) to estimate the incidence of catheter-related venous thrombosis. We categorised ultrasound diagnosis into three types: Type 0, no thrombus; Type I, thrombi around catheter without obstruction of venous flow; Type II: thrombi with obstruction of venous flow. Upon initial ultrasound examination, 25 cases (27%) were categorised as Type 0, 64 (70%) as Type I and III (3%) as Type II. Of the 64 Type-I cases, 4 cases worsened to Type II within a month, and 3 others (including 1 patient who developed pulmonary embolism) became Type II after 1 month. Of the other Type-I cases, 12 cases improved to Type 0 and 45 cases remained Type I. All 10 patients categorised as Type II underwent anticoagulant therapy and resumed their chemotherapy without exacerbations of thrombosis. In cancer patients undergoing long-term chemotherapy, there is an unexpectedly high prevalence of catheter-related UEDVT, which can be detected by ultrasound at an early stage after implantation of a CV-port. Given that cancer patients with UEDVT may have worse outcomes than those without, clinicians should consider careful monitoring for UEDVT and introducing anticoagulant therapy.