Unilateral electrode placement over the dominant hemisphere has recently been shown to avoid memory disturbance after ECT while retaining therapeutic efficacy(4, 9, 10). However, the only controls used were patients on bilateral ECT and, therefore, these studies did not conclusively show the role played by dominance. Furthermore, dominance was not established by detailed testing in any of the previous papers.The present experiment answers these questions by using as controls, patients who received unilateral ECT on their dominant hemisphere, and by the use of a special test battery to establish the preferred laterality for hand, leg and eye. Twenty-eight patients were randomly assigned to receive ECT either over the dominant or over the minor hemisphere. Electrodes were placed as described by Lancaster; otherwise standard, modified ECT was employed. Memory was tested with the Wechsler Memory Scale before the first ECT and after the fifth treatment.Drug intake was restricted during the study. Patients were chosen for ECT by their regular physician, but included in the study only if they had not received ECT within the last year, and if they had no evidence of neurological brain damage.The mean scores and standard deviations for each group of 14 patients were calculated. There was no significant difference in scores before ECT, but the second test showed a decrease of 8.1 percent in the group receiving ECT on the dominant side, while the patients with electrode placement over the nondominant hemisphere had an increase in their score of 10.6 percent.Analysis of variance for repeated measures was applied. The interaction effect of shock treatment and the dominance factor was significant beyond the .001 level. The effects of shock treatment and the dominance factor when considered individually were not significant. Thus the results conclusively show that only electrode placement over the dominant hemisphere is associated with memory loss, and that memory disturbance cannot be avoided simply by unilateral electrode placement regardless of dominance.ECT need no longer be accompanied by memory disturbance. More detailed testing for dominance, such as the battery employed in this study, can guide the psychiatrist in preventing this disturbing side effect. With the substantiation of other appropriate tests the design of this study can serve to elucidate asymmetrical localization of mental functions other than verbal memory, as well as to delineate variations to be expected in ambidextrous and left-handed patients.