Parainfluenza types 1, 2 and 3 were studied in a pediatric outpatient population from 1976 to 1992 to compare seasonal patterns over time and to define better the spectrum of illness in all ages of children caused by these viruses. Parainfluenza type 1 occurred in the fall of odd numbered years; parainfluenza type 2 was less predictable; and parainfluenza type 3 appeared yearly with peak activity in spring or summer. The parainfluenza viruses were the major cause of croup and also accounted for one-half of the cases of laryngitis and over one-third of all lower respiratory tract illness in children from whom a virus was isolated. The major clinical manifestations of infection with parainfluenza types 1 and 2 were croup, upper respiratory infections and pharyngitis; for parainfluenza type 3 upper respiratory tract infection was predominant in all age groups.The parainfluenza viruses cause appreciable respiratory morbidity each year among infants and young children. They are the major cause of croup but also produce a spectrum of diseases ranging from mild upper respiratory tract infection to bronchiolitis and pneumonia.1–3Most studies have focused on the morbidity of parainfluenza viruses in infants and young children who are hospitalized.4–6Less appreciated is the impact of parainfluenza viral infections in outpatients and in older children.The parainfluenza viruses have a striking epidemiologic pattern which has evolved over the past 30 years. In the early 1960s parainfluenza types 1, 2 and 3 were all reported to be endemic.1Subsequently parainfluenza type 1 appeared repeatedly in outbreaks in the fall of even numbered years until the early 1970s when it switched to outbreaks in the odd numbered years. Parainfluenza type 2 has been more sporadic in its appearance, and parainfluenza type 3 changed in 1978 from an endemic to more of an epidemic pattern.7,8Since 1980 the epidemiologic pattern of the parainfluenza viruses has not been described. Hence in this prospective longitudinal study conducted during 16 years we describe the evolving epidemiology of parainfluenza viruses 1, 2 and 3 and their impact on children cared for in private pediatric offices.