Pilot data suggest that in depressed patients treated with fluoxetine or venlafaxine, QEEG cordance detects changes of brain function within individual subjects as soon as two days after initiation of treatment. Changes in cordance appear to be specific (i.e., do not occur in the absence of clinical improvement), and frequently precede examiner ratings of improvement (Leuchter et al., 1997; Cook et al., in submission). In addition, patterns of cordance at baseline may indicate which patients are most likely to respond to fluoxetine or venlafaxine treatment (Cook et al., 1999).