Clinical experience suggests that patients with depression and a comorbid personality disorder (PD) may have a poorer response to electroconvulsive therapy (ECT). Only a few published studies have examined the relationship between comorbid personality disorders and response of major depression to ECT. These studies have used relatively small numbers of patients. The present study is a retrospective review of 107 inpatients with a major depressive episode referred for ECT. Patients with a clinically diagnosed PD, especially a cluster B PD, had a significantly poorer acute response to ECT than those without a PD. During the first year after treatment, ECT responders with a comorbid PD had a higher rate of relapse of depression. The retrospective study design limits the strength of conclusions that can be drawn. Nevertheless, it appears that clinically diagnosed PDs may be predictive of poor outcome in patients receiving ECT for depression. Further prospective study of the relationship between both clinically diagnosed PDs and structured interview based PD diagnoses and ECT treatment response is warranted.