Among industrialized nations, the rate of rehospitalization in the United States for respiratory syncytial virus (RSV) is approximately 30 per 1000, exceptions being noted for American Indians and Alaskan natives, two ethnic groups who tend toward higher rates of RSV hospitalization. In distinction Japan reports an admission rate of 60 per 1000 for RSV disease. Yet Japan ranks considerably lower than many of its western counterparts in premature births. Whether an RSV subtype, a new viral genotype or some other unifying characteristic exists that might explain the severity of adenovirus, parainfluenza and RSV infections in this region of Asia remains to be determined. Outcomes trials in the United States, Canada, United Kingdom, Denmark and Japan all identified crowding and exposure to tobacco smoke as significant and independent risk factors for disease severity of RSV. The epidemiology of RSV is largely consistent throughout Europe, with peak outbreaks occurring in December and January. In Europe RSV accounts for 42 to 45% of hospital admissions for lower respiratory tract infections in children younger than 2 years of age, and inpatient populations tend to be younger and to experience greater disease severity. For RSV bronchiolitis lengths of stay in European hospitals range from a low of 4 days to a high of 10 days. The Infección Respiratoria Infantil por Virus Respiratorio Sincitial Study Group in Spain conducted 2 prospective observational studies in 14 and 26 neonatal units, respectively, on nonprophylaxed neonates to determine hospitalization rates for respiratory syncytial viral illness during 2 consecutive RSV seasons. Throughout each respiratory season the study group followed premature infants of < or =32 weeks gestational age at birth, representing an annual birth cohort of approximately 100 000 infants. A total of 584 infants who were < or =32 weeks gestational age in the first season and 999 in the second season were followed at monthly intervals. The nonprophylaxed hospitalized group was compared with the nonprophylaxed, nonhospitalized group, and presumptive risk factors were determined for rehospitalization among premature infants. These independent variables, similar for both years, were identified as low gestational age, underlying chronic lung disease, living with school age siblings, exposure to tobacco smoke and a chronologic age of < 3 months at the onset of the RSV season.Stable, yet high rates of admission for RSV illness in Spain were observed in this premature group of < or =32 weeks gestational age: 13.4% for 1999; and 13.1% for the year 2000. Of those hospitalized during the 2 years of the study, 18 and 25%, respectively, were admitted to the intensive care unit. With the exception of higher rates of family allergy, multiple deliveries and a lower rate of neonatal morbidity, prognostic variables for high risk of hospital admission in the year 2000 compared with those of the 1999 sample. Findings from this comprehensive, prospective study served as the basis for the development of standards for the prevention of respiratory syncytial virus infection by the Spanish Society of Neonatology.