Introduction: Long-term dialysis catheters provide an important access for hemodialysis for patients with end-stage renal disease. They are however associated with a significant incidence of early malfunction.  The present study aims at identifYing the various causes of long term dialysis catheter early malfunction as well as the accuracy of the currently used diagnostic modalities for assessment of such causes. Patients  and methods:  51 patients (24 males 27 females, mean age 64.6 years) with early long term dialysis catheter malfunction were included in the study. All patients were evaluated by duplex ultrasonography  and magnetic resonance venography (MRV) and results ofthe two diagnostic modalities were compared. Results: Both duplex scan and MRV demonstrated central venous thrombosis in 4 patients (7.8%). Central venous attenuation or stenosis secondary to previous catheter insertion was identified in 13 cases (25.5%)  by MRV  but was only detected  in 3 cases (5.9%)  by duplex ultrasonography (p > 0.001). Conclusion: Causes of catheter malfunction in the present study included malposition, intra or peri-catheter thrombosis, and central vein attenuation  or stenosis. While duplex scan and MRV were equally  accurate in detection of central venous thrombosis, MRV  was much more sensitive in detection of central vein attenuation or stenosis. We recommend the wider use of MRV for investigating  patients with early malfunction of such dialysis catheters as well as for preoperative evaluation of patients with previous catheter insertion