Adenovirus infections have been reported in as many as one-fifth of bone marrow transplant (BMT) recipients and patients with acquired immunodeficiency syndrome (AIDS), and in a lesser, though still prominent, proportion of organ transplant recipients. The relative contributions of primary infections versus reactivations from latency in immunocompromised patients remain unclear. Compared with adult BMT recipients, pediatric BMT recipients appear to be infected by adenovirus more frequently and earlier in the post-transplant period. The diagnosis of adenovirus infection is complicated by the existence of > 40 viral serotypes, although certain subgroups are more likely to be involved in certain patient populations. Adenoviruses are responsible for a broad range of clinical diseases that may be associated with high mortality, including pneumonia, hepatitis, encephalitis, hemorrhagic cystitis, and gastroenteritis. The clinical and histopathologic features of adenovirus disease may resemble those of cytomegalovirus disease, potentially complicating the diagnosis. Risk factors for clinical adenovirus disease include the number of sites from which the virus is cultured and, in BMT recipients, the presence of moderate to severe acute graft-versus-host disease.