Background <p>Research indicates that higher levels of inflammation in patients with depression could be associated with a better response to electroconvulsive therapy (ECT). However, studies on the usefulness of immune markers as a potential biomarker for ECT response are scarce.</p> Aim <p>This study sought to investigate whether peripheral immune cell markers are associated with ECT response in patients with depression.</p> Method <p>Twenty-seven patients who received ECT treatment with a diagnosis of major depression in the Karadeniz Technical University psychiatry inpatient clinic were evaluated retrospectively. Baseline hemogram parameters of the patients, including neutrophil-to-lymphocyte (NLR), platelet-to-lymphocyte (PLR), and mean platelet volume (MPV) and C-reactive protein (CRP) levels, were determined. The relation of these markers with clinical characteristics, pre- and post-treatment Montgomery-Asberg Depression Rating Scale (MADRS) scores, and treatment response was evaluated.</p> Results <p>Baseline serum MPV levels were significantly higher in responders than non-responders to ECT (<i>p</i> = 0.002). Higher baseline MPV levels were significantly correlated with a higher decline in MADRS scores during treatment (<i>r</i> = 0.421, <i>p</i> = 0.029) and lower end-of-treatment MADRS scores (<i>r</i> = -0.489, <i>p</i> = 0.010). In multivariate regression analysis, baseline MPV levels were found to be a significant factor predicting treatment response independent of age, sex, and baseline MADRS scores. At ROC analyses, the optimum cut-off values for prediction of treatment response to ECT were 9.8 for MPV (sensitivity of 72.2%, specificity of 88.9%, <i>p</i> = 0.003).</p> Conclusion <p>MPV could be a potential biomarker for ECT response in patients with depression. Further prospective studies are now needed to verify these findings.</p>