Serum samples from 68 immunocompetent infants (mean age, 12.6 months) with an acute adenovirus infection of the respiratory tract (39 experiencing their first adenovirus infection) were tested for the presence of adenovirus DNA, to investigate whether viral dissemination via the blood is usually present in the immunocompetent patient. Using a nested polymerase chain reaction assay, adenovirus DNA could be detected in acute-phase serum samples from 28 (41%) children. Adenovirus DNA was never found in follow-up serum samples, indicating a short period ( approximately 1 week) of viral dissemination. In children experiencing their first adenovirus infection, viral DNA could be detected in 72% of the acute-phase serum samples collected within the first week after onset of symptoms. Adenovirus DNA could also be detected in 25% of the acute-phase serum samples from patients with reinfection.