Objective: To investigate fractional exhaled nitric oxide (FeNO) levels in infants during acute respiratory syncytial virus (RSV) bronchiolitis and during convalescence. Design: Prospective cohort study. Comparison of FeNO levels between infants with laboratory-confirmed acute RSV bronchiolitis and 2 control groups: healthy infants and infants with recurrent wheezing. Setting: The Department of Pediatric Emergency Medicine and the Pediatric Pulmonary Clinic of the Tel Aviv Medical Center from November 2008 to July 2009. The FeNO levels were measured at referral and at 2 visits over 4 months after convalescence. The FeNO level was measured using the multiple-breath exhalation technique. Participants: Forty-four infants with acute RSV bronchiolitis (mean [SD] age, 6.8 [7.3] months), 21 infants with recurrent wheezing (mean [SD] age, 10.8 [7.59] months), and 32 age-matched healthy controls (mean [SD] age, 6.8 [9.1] months). Follow-up data were available for 22 children (55%) for the first follow-up visit and for 11 children (25%) for the second follow-up visit. Exposure: Acute RSV bronchiolitis. Main Outcome Measures: The FeNO levels during acute RSV bronchiolitis vs controls and FeNO levels during follow-up vs acute-stage disease. Results: Mean FeNO levels for RSV-positive infants were significantly lower compared with healthy controls and infants with recurrent wheezing: mean (SD), 1.89 (1.76) parts per billion (ppb), 7.28 (4.96) ppb, and 4.86 (7.49) ppb, respectively (P<.001). The FeNO levels at the 2-and 4-month follow-up visits increased to 7.74 (5.13) ppb and 11.37 (6.29) ppb, respectively (P=.001). Conclusions: The FeNO levels are temporarily reduced during acute RSV bronchiolitis and increase during convalescence to normal levels and higher. The mechanisms for this suppression and its relation to future wheezing and asthma need to be studied.