An increase in the incidence of adenovirus (AdV) infection leading to death among children who have undergone allogeneic stem-cell transplantation has made it necessary to find new ways to monitor AdV infection. In this retrospective study, levels of AdV DNA in serum samples obtained from 36 transplant recipients with stool cultures positive for AdV were measured by polymerase chain reaction (PCR) semiquantitatively by analyzing serial dilutions of the DNA template. Six (86%) of 7 children who died of AdV infection, compared with only 2 (7%) of 29 other patients, had high serum levels of AdV DNA (detectable by PCR at a > or =100-fold dilution of the DNA template; P<.0001). High serum levels of AdV DNA were reached a mean of 18 days before death (range, 6-29 days). Quantification of adenoviral DNA in serum may prove to be a valuable tool to diagnose and monitor AdV infection and disease in immunocompromised children.