The quantitative EEGs of drug-free depressed patients were analyzed and compared to age and sex-matched controls, using spectral analysis. In addition, QEEGs of depressed patients after clinical improvement resulting from 6 weeks of antidepressant treatment were also analyzed. The subjects were 20 patients suffering from major depression (DSM-III-R). Scores on the Hamilton Rating Scale for Depression (HRSD) of all patients showed a reduction of more than 50% at the end of the 6th week. The results show: (1) delta and theta bipolar absolute powers of the right hemisphere increased in drug-free depressed patients, compared to controls. (2) No changes in all bands of QEEG were found after clinical improvement resulting from 6 weeks of treatment. These results suggest that the right hemisphere plays an important role in major depression, and that a reduction in symptoms is not necessarily indicative of an improvement in underlying major depression.