The authors review the recent, conflicting findings of the effect of ECT on diabetes mellitus. To further explore this relationship, they present case reports of three adult-onset diabetic patients with varying degrees of diabetic management who were treated for depression with ECT. The results point to the variability of effects which ECT may have on blood glucose in diabetic patients. ECT may produce a potentially dangerous hyperglycemia in severely diabetic patients, but ECT does not appear to exert a major diabetogenic or diabetolytic effect on patients with mild diabetes mellitus. Possible mechanisms by which ECT affects blood glucose levels are discussed.