Objectives- The pretransverse or first segment of the vertebral artery may be confused with adjacent branches of the proximal subclavian artery during Doppler assessment. This study investigated the effectiveness of mastoid process percussion, the mastoid tap maneuver, for identification of the vertebral artery ostium. Methods- Fifty patients presenting consecutively for carotid sonography were recruited. Doppler waveforms were collected at the vertebral artery ostia, thyrocervical trunks, and proximal subclavian arteries while the mastoid tap maneuver was performed. The outcome indicator was serrate distortion of the Doppler waveform. Two raters graded the waveforms according to a 3-grade system: grade 0, no distortion; grade 1, mild distortion; and grade 3, marked distortion. The difference between the proportions of the vertebral artery ostia and thyrocervical trunks showing waveform distortion was evaluated with the chi(2) test. The differences in the extents of waveform distortion in the ipsilateral vertebral artery ostia, thyrocervical trunks, and subclavian arteries were evaluated with Friedman and Wilcoxon signed rank tests. Results- Ninety-five vertebral artery ostia in 50 patients were successfully assessed. There was a significant difference between the proportions of vertebral artery ostia (95 of 95 [100 ) and thyrocervical trunks (9 of 95 [9.5 ) that showed waveform distortion (P <.001). There were significant differences in the extents of distortion between the ipsilateral vertebral artery ostia and thyrocervical trunks and between the ipsilateral vertebral artery ostia and subclavian arteries, with the vertebral artery ostia showing a higher degree of distortion in both cases (P <.001). Conclusions- The mastoid tap maneuver is useful for distinguishing between the vertebral arteries and thyrocervical trunks on Doppler studies.