OBJECTIVE To summarize the risk of occult serious bacterial infection in the youngest febrile infants presenting with either clinical bronchiolitis or respiratory syncytial virus infection. DATA SOURCES We performed a systematic search of the Medline database for studies reporting rates of serious bacterial infection in infants younger than 90 days with clinical bronchiolitis and/or respiratory syncytial virus infection. STUDY SELECTION Studies reporting on cultures performed at the time of presentation to care and providing a denominator, ie, total number of each type of culture obtained, were analyzed. MAIN EXPOSURE Admission for bronchiolitis. MAIN OUTCOME MEASURES Age-specific rates of urinary tract infection, bacteremia, and meningitis were extracted. RESULTS The weighted rate of urinary tract infections in the youngest infants in the 11 studies analyzed was 3.3% (95% confidence interval, 1.9%-5.7%). No case of bacteremia was reported in 8 of 11 studies. No case of meningitis was reported in any of the studies. Summary statistics for meningitis and bacteremia are not provided because of an excess of zero events in these samples. CONCLUSIONS A screening approach to culturing for serious bacterial infections in febrile infants presenting with bronchiolitis or respiratory syncytial virus infection is very low yield. The rate of urine cultures positive for bacteria remains significant, though asymptomatic bacteriuria may confound these results.