OBJECTIVES: New interventions to prevent respiratory syncytial virus (RSV) have recently become available. Clinical decisions about the use of these interventions require a better understanding of the incidence of and risk factors for RSV. We sought to characterize the epidemiology of severe RSV disease among premature infants and to identify high-risk subgroups. DESIGN: Retrospective cohort. SETTING: Kaiser Permanente Northern California, July 1992 to April 1996. PARTICIPANTS: One thousand seven hundred twenty-one premature infants born at 23 to 36 weeks who were discharged from a neonatal intensive care nursery (NICU) within 12 months before the December to March RSV season. A secondary analysis included 769 infants discharged during the RSV season. OUTCOME MEASURES: Hospitalization for RSV. RESULTS: Of 1721 infants already home from the NICU at the start of the season, 3.2% were rehospitalized for RSV. In a multivariate model, risk factors for RSV hospitalization included gestation =32 weeks (odds ratio [OR], 2.6), >/=28 days of perinatal oxygen (OR, 3.7), and NICU discharge during September to November (OR, 2.7). Predicted risk of hospitalization varied by subgroup, ranging from 1.2% to 24.6%. Among 769 infants discharged from the NICU during the RSV season, 3.5% were rehospitalized for RSV during the same season; gestation and perinatal oxygen were not associated with admission. CONCLUSIONS: Most premature infants in this population were at less risk of severe RSV disease than previous studies in other populations have suggested. Preterm infants with a lower gestational age, a prolonged perinatal oxygen requirement, and NICU discharge within 3 months of the RSV season were most likely to require hospitalization for RSV disease. Cost-effectiveness analyses are needed to help define the role of available prophylactic interventions.