We previously have shown that absolute and relative power are complementary measures of brain activity (Leuchter et al., 1993). Cordance is a new QEEG measure that combines information from both absolute and relative power measures (Leuchter et al., 1994). In validation against data collected simultaneously with 15O positron emission tomography (PET), cordance values in the theta frequency band (4-8 Hz) were found to be positively correlated with cortical perfusion, and this correlation was stronger than that between perfusion and either absolute or relative theta power (Leuchter et al., 1999). The correlation of cordance with regional cortical perfusion provides a physiologic context in which to interpret this measure.