Vasovagal syncope may be due to a transient cerebral hypoperfusion that accompanies frequency entrainment between arterial pressure (AP) and cerebral blood flow velocity (CBFV). We hypothesized that cerebral autoregulation fails during fainting; a phase synchronization index (PhSI) between AP and CBFV was used as a nonlinear, non-stationary, time-dependent measurement of cerebral autoregulation. Twelve healthy control subjects and 12 subjects with a history of vasovagal syncope underwent 10 minute tilt table testing with continuous measurement of AP, CBFV, heart rate (HR), end-tidal CO2 (ETCO2), and respiratory frequency. Time intervals were defined to compare physiologically equivalent periods in fainters and control subjects. A PhSI value of "0" corresponds to an absence of phase synchronization and efficient cerebral autoregulation, while a value of "1" corresponds to complete phase synchronization and inefficient cerebral autoregulation. During supine baseline conditions, both the control and syncope group demonstrated similar oscillatory changes in phase, with mean PhSI values of 0.58 ± 0.04 and 0.54 ± 0.02, respectively. Throughout tilt, control subjects demonstrated similar PhSI values as compared to supine conditions. Approximately two minutes prior to fainting, syncopal subjects demonstrated a sharp decrease in PhSI (0.23 ± 0.06) representing efficient cerebral autoregulation. Immediately following this period, PhSI increased sharply suggesting inefficient cerebral autoregulation and remained elevated at the time of faint (0.92 ± 0.02) and during the early recovery period (0.79 ± 0.04), immediately after return to the supine position. Our data demonstrates rapid, biphasic changes in cerebral autoregulation, which are temporally related to vasovagal syncope. Thus, a sudden period of highly efficient cerebral autoregulation precedes virtual loss of autoregulation, which continued during and after the faint.