We conducted a prospective study to evaluate the vertebrobasilar system in adults with and without obstructive sleep apnea (OSA). Our study population was made up of 48 patients with OSA and 21 healthy volunteers who served as controls; the OSA patients were subdivided into one group with mild or moderate OSA (n = 22) and another with severe OSA (n = 26). Each participant underwent Doppler ultrasonography three times to measure the diameter of the vertebral artery, the peak systolic velocity (PSV), the resistive index (RI), and the vertebral artery flow volume; the mean of the three measurements was calculated for each patient, for the OSA and control groups, and for various subgroups. No significant differences in vessel diameter, PSV, or RI were seen among any of the subgroups. Overall, the vertebral artery flow volume was slightly, but not significantly, higher in all patients with OSA (206 ml/min) than in the control group (177 ml/min); this difference might reflect the body's daytime response to the chronic apneic events experienced during sleep. The only statistically significant difference we found was in vertebral artery flow volume between the controls and the subgroup with mild or moderate OSA (p = 0.026); no difference was seen between the controls and the patients with severe OSA (p = 0.318). Likewise, no significant difference in any of the four parameters was seen when patients were subclassified by body mass index and arterial oxygen saturation level.