The aim of this study was to compare the clinical course of severe lower respiratory tract infections associated with human parainfluenza virus types 1-3 (HPIV 1-3) in hospitalised children infected with the human immunodeficiency virus type 1 (HIV-1) versus that in hospitalised children not infected with HIV-1. Children were enrolled prospectively as part of a broader study that evaluated the aetiology of lower respiratory tract infections in HIV-1-infected and -noninfected children from March 1997 through March 1999. HPIV types 1-3 were isolated from nasopharyngeal aspirate samples that were analysed using immunofluorescein monoclonal antibody assays. Thirty percent (24 of 80) of the children from whom HPIV was isolated were infected with HIV-1. Sixty-six percent (47 of 62) and 22% (14 of 62) of the HPIV isolates that were typed were subtypes 3 and 1, respectively. The clinical presentation of severe lower respiratory tract infection was similar in both HIV-1-infected and -noninfected children, except that the former were less likely to have wheezing (4.2% vs. 28.6%, P=0.01). Furthermore, the duration of hospitalisation was longer in HIV-1-infected children than in HIV-1-noninfected children (median 11.5 days [range 1-15 days] vs. median 7.5 days [range 1-22 days]; P=0.02), and mortality was higher (5 of 24 [20.8%] infected children vs. 0 of 56 noninfected children; P=0.001). Importantly, four of five (80%) of the HIV-1-infected children who died had other concurrent illnesses or predisposing factors for severe HPIV-associated disease. HPIV-associated lower respiratory tract infection causes greater morbidity and mortality in HIV-1-infected children than in HIV-1-noninfected children; however, this may be due to other concurrent illnesses in HIV-1-infected children.