Croup is a common manifestation of respiratory tract infection in children, and human parainfluenza virus 1 (HPIV-1) is the agent most commonly associated with croup. In the United States, HPIV-1 produces a distinctive pattern of biennial epidemics of respiratory illness during the autumn months of odd-numbered years. National Hospital Discharge Survey data for croup hospitalizations among patients <15 years old between 1979 and 1993 were examined along with laboratory-based surveillance data on HPIV-1 activity in the United States. The mean annual number of croup hospitalizations was 41,000 (range, 27,000-62,000/year). Ninety-one percent of hospitalizations occurred among children <5 years of age. Minor peaks in croup hospitalizations occurred each year in February, and major peaks occurred in October of odd-numbered years, coincident with peak HPIV-1 activity. Each biennial epidemic of HPIV-1 was associated with 18,000 excess croup hospitalizations nationwide.