BACKGROUND For children hospitalized with bronchiolitis, there is uncertainty about the expected inpatient clinical course and when children are safe for discharge. OBJECTIVES Examine the time to clinical improvement, risk of clinical worsening after improvement, and develop discharge criteria. DESIGN Prospective multiyear cohort study. SETTING Sixteen US hospitals. PARTICIPANTS Consecutive hospitalized children age <2 years with bronchiolitis. MEASUREMENT We defined clinical improvement using: (1) retraction severity, (2) respiratory rate, (3) room air oxygen saturation, and (4) hydration status. After meeting improvement criteria, children were considered clinically worse based on the inverse of ≥1 of these criteria or need for intensive care. RESULTS Among 1916 children, the median number of days from onset of difficulty breathing until clinical improvement was 4 (interquartile range, 3-7.5 days). Of the total, 1702 (88%) met clinical improvement criteria, with 4% worsening (3% required intensive care). Children who worsened were age <2 months (adjusted odds ratio [AOR]: 3.51; 95% confidence interval [CI]: 2.07-5.94), gestational age <37 weeks (AOR: 1.94; 95% CI: 1.13-3.32), and presented with severe retractions (AOR: 5.55; 95% CI: 2.12-14.50), inadequate oral intake (AOR: 2.54; 95% CI: 1.39-4.62), or apnea (AOR: 2.87; 95% CI: 1.45-5.68). Readmissions were similar for children who did and did not worsen. CONCLUSIONS Although children hospitalized with bronchiolitis had wide-ranging recovery times, only 4% worsened after initial improvement. Children who worsened were more likely to be younger, premature infants presenting in more severe distress. For children hospitalized with bronchiolitis, these data may help establish more evidence-based discharge criteria, reduce practice variability, and safely shorten hospital length-of-stay.