Background: The use of a central venous catheter as a permanent vascular access is constantly growing. The recirculation rate in this type of vascular access varies depending on the site of insertion, the length of the catheter, the blood flow and the time elapsed since catheter insertion. When the in/out flow of the lumens of the catheter is reduced, it is sometimes necessary to inverse the arterial and venous lines of the catheter at the beginning or in the course of the dialysis session. The impact of such a practice on the recirculation rate has only been assessed by the low flow urea dilution method. Methods: The blood recirculation rate was measured using the ultrasound dilution velocity method (Transonic<sup>®</sup> system, Ithaca, N.Y.) in 14 patients (aged 64 ± 15 years), with a right internal jugular (n = 9) or right subclavian (n = 5) central catheter (Twincath, Medcomp<sup>®</sup>), used over a mean period of 16.3 (range 1–42) months. No clinical dysfunction of the catheter was apparently noted. The distance between the vein and the artery end of the catheter was 3.9 ± 2.8 cm, measured on chest radiography. The recirculation rate was measured over a single dialysis session at increasing blood flows in the normal and inversed position of the catheter. Results: The difference between the prescribed and effective blood flow was significant with an effective blood flow of 180± 16 ml/min for 200 ml/min, 264 ± 27 ml/min for 300 ml/min and 329 ± 16 ml/min for 400 ml/min prescribed blood flow. There was no relationship between the recirculation rate and blood flow whatever the position of the lines on the catheter. There was also no relationship between the recirculation rate and the distance between the catheter ends. However, reversing the catheter ends significantly increased the recirculation rates from 2.9 ± 5 to 12 ± 9% whatever the blood flow. Conclusion: The use of central catheters in an inversed position can result in a higher recirculation rate. Reversing the lines of the central catheters may lead to less effective hemodialysis and it seems particularly important for the nursing staff to be aware of this phenomenon. Thus, measurement of the effective blood flow and recirculation by ultrasound velocity should be included in quality monitoring and maintenance.