Among 48 patients with diagnoses of depression according to DSM-III, there was a significant relation between therapeutic failure of unilateral ECT, as measured by scores on the Hamilton Rating Scale for Depression, and the concomitant use of a benzodiazepine. Of the 34 patients who showed a good therapeutic response to unilateral ECT, those taking benzodiazepines had smaller changes in their Hamilton depression ratings from before treatment to after treatment and were more symptomatic at the end of the course of ECT. Thus, when patients take benzodiazepines during a course of unilateral ECT, the maximum therapeutic response may be compromised.