The use of electroconvulsive therapy (ECT) in mental retardation has been discussed in several case reports and case series. In this case, a 35-year-old patient with corpus callosum aplasia and severe therapy-resistant catatonia was treated with a series of unilateral ECT and improved considerably. Electroencephalographic recordings during ECT showed a complete interhemispheric synchronicity due to regular anterior and posterior commissural fibers. After ECT, the patient received long-term medication with quetiapine and lorazepam. Electroconvulsive therapy turned out to be a powerful tool for treating catatonic syndromes in patients with mental retardation and should be considered as a potent treatment option in otherwise therapy-resistant cases.