BACKGROUND: Hemodynamic changes accompany the initiation, development and rupture of middle cerebral artery (MCA) aneurysms. The complexity of the intraaneurysmal hemodynamic factors has not been completely clarified by the indirect measures and methods used in previous studies. OBJECTIVE: To evaluate correlations of intraoperative hemodynamic factors to initiation and rupture of MCA aneurysms. DESIGN, TIME AND SETTING: A case-control study was performed at the Department of Neurosurgery, Tiantan Hospital Affiliated to Capital Medical University, China between March and October 2008. PARTICIPANTS: A total of 12 consecutive patients diagnosed with MCA aneurysms (MCA aneurysms group) and five patients without middle cerebral artery aneurysms (with aneurysms located at other arteries, control group) were enrolled at the Department of Neurosurgery, Tiantan Hospital Affiliated to Capital Medical University, China. METHODS: The proximal and distal arteries of MCA aneurysms were exposed visibly in the MCA aneurysm group. The M1 segment of MCA without the aneurysm and the aneurysm on other arteries were also exposed visibly in the control group. Hemodynamic indices were then measured using an intraoperative 16 MHz probe installed in a Multi-Dop TCD8X4 device. MAIN OUTCOME MEASURES: Mean (time-averaged velocity) difference, maximum mean, pulsatility index difference, maximum pulsatility index, resistance index difference, maximum resistance index; correlation of development and rupture of MCA aneurysms to intraoperative hemodynamic factors of the parent artery. RESULTS: A total of 12 patients underwent microsurgery for treatment to occlude 15 MCA aneurysms. Of the 15 MCA aneurysms, 12 were located at the bifurcation, two at the M1 segment, and one at the M3 segment; eight of the aneurysms were unruptured and seven were ruptured. The whole indices with combination mean difference, maximum mean, and maximum pulsatility index of the aneurysms were important factors influencing the rupture of MCA aneurysms (t = 2.92, P = 0.03, constant). A higher velocity intraaneurismal flow at the bifurcation was identified (t = 3.48, P = 0.01, constant). After the aneurysm was completely occluded, global high-velocity flow could not be detected in the parent arteries (t = 2.57, P = 0.03, constant). CONCLUSION: When short-term high-velocity blood flow is present, aneurysms can be easily initiated and ruptured at the bifurcation of MCA.