Vitamin B-12 deficiency is a worldwide problem. It affects all ages, including children. It is one of the most common nutritional disorders and can cause harmful effects on the nervous system. In this study, we compared the peripapillary retinal nerve fiber layer thickness (RNFLT) in a healthy control group with children with vitamin B-12 deficiency. In our study, we aimed to evaluate the effect of vitamin B-12 deficiency on the RNFLT in children with the optical coherence tomography (OCT) method.