Objective: Lithium is a drug of choice in the treatment of bipolar disorder and refractory depressive disorders. However, previous research suggests lithium has a negative cognitive impact in recovery from electroconvulsive therapy (ECT) and a higher risk of delirium, so patients are often required to stop taking lithium before ECT, despite risk of relapse.We studied the cognitive impact of serum lithium levels in patients undergoing ECT. Methods: This was an observational prospective study. Serum lithium levels, thyroid and biochemical parameters were measured prior to each ECT session. Time elapsed from the anesthetic induction to the electrical stimulus and then to the patients’ reorientation was recorded, as well as the motor seizure duration and electroencephalogram (EEG) seizure duration. A statistical analysis using a linear mixed model was run while adjusting for confounding factors. Results: Ten participants underwent a total of 86 ECT sessions (41% right unilateral ultrabrief pulse, and 59% bilateral brief pulse). A negative interaction between lithium levels and reorientation time was found among those doing bilateral brief pulse ECT. No association was observed in patients doing unilateral ultrabrief pulse ECT. No significant relationship was observed between lithium and both motor and EEG-assessed seizure duration. Conclusion This study suggests that low to moderate serum lithium levels (＜ 0.7 mmol/L) might have no harmful cognitive effects in patients under right unilateral ultrabrief pulse and bilateral brief pulse ECT.