Adenovirus infection is second only to rotavirus as a cause of childhood diarrhea and is associated with a broad spectrum of clinical disease in both children and adults. However, it has gained attention in recent years as a cause of diarrhea in immunocompromised patients, especially those with AIDS and patients who have undergone bone marrow or solid organ transplants. The incidence of adenovirus infection in bone marrow transplant patients has been reported at 5–20%; the incidence is lower in solid organ transplant patients. Pediatric transplant patients are affected more often than adults. Patients with severe combined immunodeficiency are also susceptible to gastrointestinal adenovirus infection. Virtually all patients present with diarrhea, sometimes accompanied by fever, weight loss, and abdominal pain. Risk factors for disseminated adenovirus infection include presence of concomitant graft-versus-host disease, use of immunosuppressive therapy, isolation of virus from multiple sites, and HLA-mismatched or unrelated transplants.