We report 2 cases of adenovirus enterocolitis in pediatric patients who underwent bone marrow transplantation. The first case involved a 17-year-old adolescent boy with combined immunodeficiency and non-Hodgkin lymphoma who developed chronic graft versus host disease and persistent adenovirus duodenitis. Case 2 involved a 3-year-old boy who received a mismatched unrelated bone marrow transplant for metachromatic leukodystrophy; the boy developed severe graft versus host disease and died of multiorgan failure. At autopsy, diffuse hemorrhagic enterocolitis with changes of severe graft versus host disease and extensive mucosal invasion by adenovirus was found. Awareness and early recognition of this uncommon complication of concomitant graft versus host disease and adenovirus infection could impact therapy and outcome of patients with bone marrow transplant.