Background:Although human parainfluenza virus (HPIV) has been determined as an important viral cause of acute respiratory infections (ARIs) in infants and young children,data on long-term investigation are still lacking to disclose the infection patt of HPIV in China.Methods:Nasopharyngeal aspirates were collected from 25,773 hospitalized pediatric patients with ARIs from January 2004 through December 2012 for respiratory virus screen by direct immuno-fluorescence assay.Results:Out of these specimens,1675 (6.50％,1675/25,773) showed HPIV positive,including 261 (1.01％,261/25,773) for HPIV1,28 (0.11％,28/25,773) for HPIV2,and 1388 (5.39％,1388/25,773) for HPIV3,2 of the samples were positive for both HPIV1 and HPIV3,and 36 were co-detected with other viruses.The positive rates of HPIVs were higher in those younger than 3 years old.HPIV3 was detected from all age groups,predominantly from patients under 3 years of age,and the highest frequency was found in those 6 months to 1-year old (352/4077,8.63％).HPIV3 was the dominant type in each of the years detected between May and July.HPIV1 showed a peak in every odd year,mainly in August or September.HPIV was detected most frequently from patients with upper respiratory infection (12.49％,157/1257),followed by bronchitis (11.13％,176/2479),asthma (9.31％,43/462),bronchiolitis (5.91％,150/2536),pneumonia (6.06％,1034/17,068),and those with underlying diseases (1.0％,15/1506).HPIV3 is the dominant type in these six disease groups referred above,especially in the asthma group.Conclusions:HPIV is one of the important viral causes of ARIs in infants and young children in Beijing based on the data from the hospitalized children covering a 9-year term.HPIV3 is the predominant type in all these years and in most of the disease groups.HPIVs with different types show different seasonality.