Aim: This study was conducted to evaluate the influence of vitamin B-12 replacement therapy on event-related potentials (ERPs) and cognitive functions of the patients with vitamin B-12 deficiency. Material and Method: Patients were divided into three groups according to the serum vitamin B-12 levels of individuals. Sixteen patients with serum vitamin B-12 levels %u2264197 pg/ml and with complaints of cognitive dysfunction were enrolled in the group 1. Group 2 (n=17, with serum vitamin B-12 levels between 198-350 pg/ml) and group 3 (n=16, with serum vitamin B-12 levels above 350 pg/ml) were formed by healthy individuals. Standardized mini mental state examination (SMMSE) test was performed to all individuals. Also the SMMSE test was applied to group 1 again 3 months after the treatment. In group 1, to measure the effectiveness of vitamin B-12 replacement therapy, ERP studies were performed before and after treatment. Results: In group 1, P300, N200 and P200 amplitudes were found to be increased, and P300, N200 and P200 latencies were found to be shortened after the treatment. Discussion: Study results suggest that improvement in P300, N200 and P200 amplitudes and latencies together with SMMSE scores are useful parameters in the assessment of vitamin B-12 replacement therapy.