Aim: To investigate the impact of different spectral Doppler criteria on the proportion of high-grade ICA stenosis in patients undergoing carotid artery duplex scanning. Material and methods: Duplex scans of 4,548 internal carotid arteries (ICA) in 2,349 patients were retrospectively analyzed. The following different criteria were applied for each scan for definition of ICA stenosis ≥ 70%: Criteria I = ICA peak systolic velocity (PSV) > 130 cm/sec and ICA end-diastolic (EDV) > 100 cm/sec, Criteria II = PSV ICA/common carotid artery (CCA) ratio > 4, Criteria III = ICA PSV ≥ 230 cm/sec, Criteria IV = ICA PSV > 230 cm/sec and/or ICA EDV ≥ 100 cm/sec and/or PSV ICA/CCA ratio ≥ 3.2. Results: The frequency of detecting a ≥ 70% ICA stenosis with criteria I, II, III, and IV were 5.5%, 6.8%, 8.4%, and 9.6%, respectively (p < 0.05). Conclusion: The use of various duplex criteria significantly affected the number of scans receiving a diagnosis of ICA stenosis of ≥ 70%.