Objectives: Severe depression is associated with an increased risk of developing dementia, however, whether treatment with electroconvulsive therapy (ECT) modify this risk remains unknown. Methods: In this matched cohort study, 1089 consecutive in-patients with affective disorders, receiving ECT during the period 1982 to 2000, were matched with 3011 in-patients with affective disorders not treated with ECT (non-ECT), and 108,867 individuals randomly selected from the background population. The comparison cohorts were matched on sex, age, and the non-ECT cohort was further matched according to diagnoses and admission period and hospital. Dementia diagnoses were retrieved from the national patient health registry. Analyses were adjusted for disease severity, somatic, and psychiatric comorbidities. Results: The cumulative incidence of dementia was 13.45% (10.75-16.46%) in the ECT cohort after 34 years of follow-up, 10.53% (8.5-12.81%) in the non-ECT cohort, and 8.43% (8.17-8.7%) in the background cohort. Using the ECT cohort as reference and age as the underlying time scale, the adjusted hazard ratio of developing dementia was 0.73 (0.52-1.04) in the non-ECT cohort and 0.61 (0.49-0.76) in the background cohort. The stratified analysis based on age at index (<65 years; 65-80 years; >80 years) found no age-related difference in the risk of developing dementia between the ECT cohort and non-ECT cohort. Conclusions: The ECT treatment of affective disorders was not associated with an increased long-term risk of developing dementia compared with in-patients with affective disorders not treated with ECT.