Serum levels of ascorbic acid and vitamin B12 were analyzed in 40 myelomeningocele children to study the effect of supplemental ascorbic acid on serum vitamin B12 levels. The experimental group was composed of 20 children receiving ascorbic acid for urinary acidification: 10 received an average of 1.8 g daily, 10 received an average of 1.5 g daily (the amount depending on the requirement needs for urinary acidification) half of each group received ascorbic acid for less than 3 yr (an average of 2.1 yr) and half received ascorbic acid for more than 3 yr (an average of 4.3 yr). The control group consisted of 20 myelomeningocele children not receiving supplemental ascorbic acid. Both groups were matched for age, sex, race, and physical activity. Dietary levels of ascorbic acid and B12 were calculated to rule out their influence on serum levels. Results showed that the experimental group with supplemental ascorbic acid produced significantly higher ascorbic acid values than the control group. The serum B12 levels of the experimental group were not significantly different than those of the control groups and these children showed neither a deficient serum levels of B12, anemia, nor elevated mean corpuscular volume. Hemoglobin levels were slightly higher for the experimental group. Dietary calculations of B12 and ascorbic acid were not significantly greater than the Recommended Daily Allowance ruling out any influence of diet on serum levels. No evidence of vitamin B12 deficiency developed in 20 myelomeningocele children receiving daily mean doses of 1.65 g of supplemental ascorbic acid. In view of our findings, it is highly improbable that megadoses of supplemental ascorbic acid would induce vitamin B12 deficiency in man.