Human parainfluenza viruses (HPHVs) are common viral pathogens of lower or upper respiratory tract infections for humans in any age, especially in infants and young children.Because of the difficulties in isolating virus from clinical specimens and the lack of routine diagnostic test, the clinical significance of human parainfluenza virus type 4 (HPHV-4) is not well defined as that ofparainfluenza virus types 1-3.To investigate the impact of HPHV-4 to acute respiratory infeehons in infants and young children in Beijing, specimens collected from 3595 children with acute respiratory infections from Mar 2005 to May 2008 were analyzed for HPHV by multiplex RT-PCR to amplify the hemagglutinin (HA) gene fragment for typing of HPHV 1-4.Then the encoding-regions of the F and HA gene of HPHV-4 were amplified from 8 HPHV-4 virus-positive clinical specimens and sequence analysis was performed.Fifty-eight out of these 3595 (1.6%) specimens from patients with ARH showed PCR products with predicated molecular weight particular for HPHV-4, including six also positive for another type HPHV, such as HPHV-2 and HPHV-3.