Introduction: Major depressive disorder (MDD) is a common psychiatric disorder. Despite several treatment options, a subgroup of patients will not respond to the commonly used antidepressant treatments and thus express treatment resistance (TRD). TRD can be quantified with the Dutch Measure for Treatment Resistance in Depression (DM-TRD). Electroconvulsive therapy (ECT) is an effective treatment for MDD, also in TRD. Yet, the position of ECT as "treatment-of-lastresort" may decrease the likelihood of beneficial outcome. Our aim was to investigate the association between treatment resistance and outcome and course of ECT.Methods: We performed a retrospective, multicenter cohort study with 440 patients of which data was retrieved from patient records as collected in the Dutch ECT Cohort database. Linear and logistic regression models were used to explore the association between level of treatment resistance and outcome of ECT. Median split was used to explore the differences between high and low level of TRD and course of treatment. Results: A higher DM-TRD score was associated with significantly smaller reduction of depression symptoms (R 2 = 0.160; β = À2.968; p < 0.001) and lower chance of response (OR = 0.821 [95 CI: 0.760-0.888]; β = À0.197; p < 0.001). Low level TRD patients underwent fewer ECT sessions (mean 13 ± 6 SD vs. 16 ± 7 SD; p < 0.001) and fewer switches from right unilateral tot bifrontotemporal electrode placement (29% vs. 40%; p = 0.032). Conclusion:Reserving ECT as "treatment-of-last-resort" in the treatment algorithm for MDD seems questionable, because in our study lower level of treatment resistance predicted more beneficial ECT-outcome. Moreover, providing ECT in less treatment resistant patients showed fewer needed ECTsessions and less switches to BL electrode placement, which may decrease the risk for cognitive side-effects.