OBJECTIVES:To measure the duration of illness in ambulatory children diagnosed with bronchiolitis and to examine clinical predictors of duration of illness. DESIGN:Validation inception cohort study. Duration of follow up was 28 days. SETTING:A primary-level ambulatory department of a public sector children's hospital in Cape Town, South Africa. PATIENTS:One hundred eighty-one children aged 2 to 23 months who went to the hospital as their first contact for that episode of illness, and had a clinical diagnosis of bronchiolitis were enrolled consecutively on weekday mornings if their guardian stated that they were contactable by telephone. MAIN OUTCOME MEASURE:Resolution of symptoms, as judged by the guardian, measured by twice-weekly telephone interviews. RESULTS:Median duration of illness (calculated as the reported duration of symptoms before initial hospital visit plus the time from first consultation to recovery) was 12 days (95% confidence interval, 11-14 days). After 21 days, 18% were still ill and after 28 days, 9% were still ill. Sixty-two patients (34.2%) had unscheduled consultations within 28 days, a median of 13 days after the first consultation. There was no association of duration of illness with age, sex, z score for weight for age, or respiratory rate. CONCLUSIONS:Ambulatory children diagnosed with bronchiolitis recover with few complications, but the resolution of symptoms may take several weeks. Providing parents with this information could help reduce the high rate of unscheduled return visits as observed in this cohort.