The investigators hypothesize that single oral high dose supplementation with vitamin A will reduce the incidence of moderate-severe chronic graft-versus-host disease (GVHD) compared with placebo.

Hematopoietic stem cell transplantation (HSCT) is an effective treatment strategy for many malignancies, marrow failure syndromes, and immune deficiencies in children, adolescents, and adults. Vitamin A and its derivatives regulate growth and differentiation of intestinal cells, and vitamin A deficiency is associated with increased susceptibility to infection in both human and animal models. The investigators' preliminary data suggest lower vitamin A levels were associated with an increased incidence of gastrointestinal graft versus host disease (GI GVHD) in patients undergoing HSCT.

This study is a randomized double blinded comparison of vitamin A supplementation comparing a single large dose of vitamin A with a placebo.

Inclusion Criteria:

• Be 18 years of age or older
• Be scheduled for allogeneic stem cell transplant.
• Have a vitamin A level < upper limit of normal for age.
• Be able to tolerate enteral vitamin dose administration.
• Have a total bilirubin level < 1.5x ULN and an AST and/or ALT< 3xULN for age
• Receiving PBSCs as stem cell graft

Exclusion Criteria:

• Ongoing raised intracranial pressure
• Liver cirrhosis
• Patients will be excluded if they are currently pregnant.