The increasing endovascular treatment of arterial stenoses requires reliable sonographic criteria for the detection of recurrent stenosis. Placement of a stent reduces the vessel lumen and is assumed to lead to greater rigidity, thereby causing more pulsatile blood flow and a higher peak systolic velocity (PSV). In vitro experiments and clinical trials with duplex ultrasound follow-up after percutaneous transluminal angioplasty (PTA) of the internal carotid artery with carotid artery stenting (CAS) provide inconsistent data on the magnitude of the increase in PSV of in-stent restenosis. They range from unchanged PSV up to 30% higher increases in PSV measurement values compared to those established for stenosis grading in native carotid stenoses. In stented leg arteries, studies even show somewhat lower PSV values compared with stenosis in unstented vessels. The option of applying the continuity law, so far not mentioned in the international literature, allows more accurate grading of in-stent restenosis. This method relates the PSV measured at the site of in-stent restenosis to the prestenotic PSV measured within the stent, thereby eliminating the need to take altered hemodynamics in stented arteries versus unstented arteries into account.