An investigation of the epidemiology and pathogenesis of bronchiolitis due to parainfluenza virus (PV) was carried out. Bronchiolitis due to PV occurred most commonly in non-Caucasian males. Breast-fed infants exhibited a reduced risk of developing bronchiolitis. Once an episode of PV bronchiolitis occurred, both exposure to cigarette smoke and bottle feeding were associated with an increased frequency of recurrent wheezing, and subsequent infection with respiratory virus almost uniformly resulted in wheezing. Cell-mediated immune responses to PV antigen and titers of PV-specific IgE were greater among patients with bronchiolitis than among patients with upper respiratory tract infection. The epidemiology and pathogenesis of bronchiolitis due to PV is similar to that of respiratory syncytial virus. Lower respiratory tract infection may predispose to episodes of bronchoconstriction on subsequent exposure to cigarette smoke or other viral infections.