Vitamin C deficiency in developed countries is typically observed in patients with unique clinical conditions such as cystic fibrosis or anorexia nervosa, or in patients on long-term tube feeds. We report here a clinical observation in six pediatric and adolescent patients (median age 17.5 yr, range 9.8-23.5 yr) with chronic GVHD with mucous membrane involvement found to be vitamin C deficient. These patients' baseline serum vitamin C levels ranged from <0.12 to 0.94 mg/dL (normal value 0.20-1.90 mg/dL), with a mean level 0.56 +/- 0.36 mg/dL and a median level 0.6 mg/dL. Among these patients, signs and symptoms of mucositis failed to respond to standard chronic GVHD therapy. After receiving treatment with 2000 mg of ascorbic acid by mouth, daily patients displayed increased serum vitamin C levels. Clinically, this correlated with a remarkable improvement in patients' mucositis and ability to eat.