The objective of this study was to assess prospectively the frequency of concurrent serious bacterial infection (SBI) in febrile infants < or = 3 months of age with or without bronchiolitis. SBI was detected in 30 of 312 (9.6%) infants without bronchiolitis compared with 3 of 136 (2.2%) infants with bronchiolitis. The risk of SBI in febrile infants is significantly lower in the presence of bronchiolitis.