The objective is to develop a clinically and ethically supportive literature for the use of electroconvulsive therapy (ECT) in patients with mental retardation who are concurrently experiencing a significant psychiatric illness. A review of both the clinical and ethical literature using traditional, manual library methods and the Medline and Psychlit databases was undertaken. In addition, a record of all patients who had undergone ECT at our facility between 1995 and 2000 was examined for patients with comorbid mental retardation. We found that the use of ECT for people who have both a psychiatric illness and comorbid mental retardation was significantly delayed. However, a rapid response to index, continuation, and maintenance ECT was also noted. Further, this response occurred with routine ECT administered irrespective of age, gender, diagnosis, stimulus parameters, electrode placement, or number of treatments. In addition, the successful use of right-sided unilateral ECT at six times the initial seizure threshold was reported in a patient who had previously responded to bilateral ECT. A cogent ethical justification was developed with the use of the rule of double effect. We concluded that for patients who have mental retardation and who subsequently develop a psychiatric illness, ECT is delayed and left as a treatment of last choice. Although the literature is sparse and uncontrolled, a cogent clinical and ethical justification may help negotiate these and other delays.