There are reports that the second most causative viral agent which causes lower respiratory tract infection (LRTI) in young children is adenovirus (ADV). Human herpes virus 6 (HHV-6) is also reported as a rare agent of LRTI in young children. But there is no report of simultaneous detection of ADV and HHV-6 in LRTI using the same peripheral blood monocyte (PBM) by nested-polymerase chain reaction (PCR) or PCR. Firstly, we detected ADV antigen (Ag) and HHV-6 Ag in serum by each monoclonal antibody with enzyme immunoassay (EIA). Secondly we tested two viruses in peripheral blood monocyte by nested-PCR or PCR. Twenty nine cases of young hospitalized children with LRTI (mean age 11.3 months, mean hospitalization period 5.7 days) had bronchiolitis or viral pneumonia and were confirmed by X-ray findings. Positivity of ADV Ag in serum by EIA was 75% (21/28) and positivity of HHV-6 Ag in serum by EIA was 10.7% (3/28). ADV in PBM by nested-PCR positivity was 89.7% (26/29) and HHV-6 in PBM by PCR positivity was 42.9% (12/28). ADV and HHV-6 dual infection in PBM by PCR was 11/29 (37.9%). Young children with dual infection were hospitalized (mean 6.3 days) with severe bronchiolitis.