In a double-blind, randomized controlled study of electroconvulsive therapy (ECT) in patients with major depression, 7 of the 17 patients allocated to the right unilateral group failed to respond to treatment. The nonresponders were subsequently openly treated with bitemporal treatment, which produced an acceptable outcome in these cases of right unilateral treatment failure. This paper describes the clinical outcome, electrophysiological characteristics (impedence, estimated seizure threshold, and change in threshold), and the degree to which stimuli exceeded threshold in the responder and nonresponder groups. Responders had lower seizure thresholds and longer seizures than nonresponders. In comparison with nonresponders, responders showed trends toward greater impedance and treatment at a somewhat greater degree above threshold during the first few treatments. Threshold change with treatment was found not to be related to clinical outcome. Early identification of patients likely to respond to low-dose right unilateral ECT, together with the avoidance of benzodiazepine prescription during ECT, may permit many patients to receive low-dose right unilateral ECT successfully and with a minimum of cognitive impairment.