Additional nutritional deficiencies may accompany zinc deficiency. We determined the vitamin B-12 levels in patients with zinc (Zn) deficiency. This retrospective study enrolled 256 patients age 6 months to 16 years (176 females and 80 males) in whom Zn levels in hair samples were measured concurrently with serum vitamin B-12 and ferritin levels. For all patients, we retrospectively assessed the hair Zn levels, serum ferritin and vitamin B-12 levels, red blood cell count, hemoglobin, hematocrit, mean corpuscular volume, mean corpuscular hemoglobin, mean corpuscular hemoglobin concentration, and red cell distribution levels. Data were analyzed to determine whether there was a significant difference between any of these parameters and the presence of vitamin B-12 deficiency between patients with and without Zn deficiency. In all 118 patients had Zn levels < 100 mu g/g and 138 patients had levels > 100 mu g/g. No significant differences were observed in ferritin levels, red blood cell parameters, or presence of iron deficiency or anemia between the two groups (p > 0.05). The median vitamin B-12 level was 323 (range 238-440) pg/mL in the Zn-deficient group and 276 (range 208-382) pg/mL in those with normal Zn levels. Vitamin B-12 levels were significantly higher in the Zn-deficient group (p = 0.02). A significant negative correlation was detected between vitamin B-12 levels and Zn levels (r = -0.17, p = 0.004). Vitamin B-12 levels are higher in patients with Zn deficiency; thus, Zn may have a negative effect on vitamin B-12 levels.