This study attempted to clarify the effect of asymmetric COW variants on carotid flow changes and proposed an easy estimate of the representative carotid flow volume. In the present study, 52 of the 210 cases (34.7%) had bilateral A1 asymmetry ≥50%, in whom there was significantly greater dominant ICA flow with larger dominant ICA diameter, and lower non-dominant ICA flow with smaller non-dominant ICA diameter, as compared with the symmetric group. There was a moderately positive association between ICA vessel diameter and ICA flow volume in the AA groups, with a 1 mm increment in vessel diameter corresponding to a 62.6 ml increment of flow volume. Moreover, an ICA-PDF cutoff value of 4.31 was found to be a useful predictor of bilateral A1 asymmetry ≥50%.