Recently developed devices provide detection of access recirculation (AR) and cardiopulmonary recirculation (CPR) by optical, thermal, conducimetrical, and ultrasound methods (USM). We evaluated the last one both in vitro reproducing AR by a bypass pump and in vivo. In vitro, the USM sensitivity was about 5%. In vivo, the USM was compared with the traditional urea method (UM) in 69 patients. 8.7% of the cases resulted positive by both UM and USM. One case was USM positive and UM negative. The UM sensitivity threshold was 6–10%. The accuracy (in vitro) and the repeatability (in vivo) of the USM were satisfactory. USM clearly distinguished AR from CPR. In conclusion, AR determination by USM, avoiding misleading interferences with CPR, is a rapid, easy, and noninvasive method to routinely exclude a potential cause of reduced dialytic efficiency.