To determine the morbidity, costs, and epidemiological features of lower respiratory tract infections (LRIs) due to human parainfluenza virus types 1 and 2 (HPIV-1 and HPIV-2), we evaluated 1,213 children < 6 years of age who were seen for LRIs in the emergency room of the Children's Hospital of Wisconsin and/or were admitted to the hospital for LRIs during the fall quarter of 1991. The age, sex, race, and respiratory syndrome were recorded for each child; 158 patients (13%) had respiratory samples cultured for viruses and were followed clinically for the duration of their illness. Caucasian children had croup diagnosed more often than did African-American children (relative risk [RR] = 3.12; 95% confidence interval [CI], 2.43-4.00; P < .001), while African-American children more often had pneumonia (RR = 1.85; 95% CI, 1.36-2.5; P < .001). Forty-five of 70 viruses recovered were HPIV-1 (17 cases) or HPIV-2 (28 cases). Together these two viruses were recovered from 49% of children presenting with croup, 10% of those presenting with bronchiolitis, and 12% of those presenting with pneumonia. Gender- and race-associated differences were documented in the group of children infected with HPIV-2: specifically, this group included more girls than boys (RR = 1.99; 95% CI, 1.02-3.88; P < .04) and more Caucasian than African-American children (RR = 2.64; 95% CI, 1.05-6.63; P = .027). These data extrapolate nationally to approximately 250,000 emergency-room visits and approximately 70,000 hospitalizations due to HPIV-1 and HPIV-2, with a cost of $50 million for the former and $140 million for the latter.