Background: Bronchiolitis usually occurs during the first year of life, with a peak incidence between 3 and 6 months of age. Chest Ultrasound has been proposed as a diagnostic tool in many diseases in the pediatric and neonatological settings. The aim of the present study was to differentiate simple versus complicated cases of bronchiolitis in infant by chest ultrasound. Patients and methods: A prospective observational study was conducted in Pediatric Department, Zagazig University Hospitals.Inclusion criteria included infants aged 1 month to 2 years fulfilling clinical criteria of acute bronchiolitis. Steps of performance included consent from relatives to participate in the study, complete history taking, full clinical examination, laboratory investigations, chest X-ray, chest Ultrasound and chest CT. Results: Our results showed that wheeze was the commonest presentation (100.0%) of the studied group followed by crepitation (12.3%) of them then Refusal of feeding (9.2%) and lastly fever (5.0%) of the studied group. There was (55.4%) of cases were complicated bronchiolitis by U/S and (44.6%) were simple. The commonest cases (96.9%) were discharged with only (3.1%) death rate. The X-ray was able to detect only (50.0%) of complicated cases and 29 cases (100.0%) were excluded by both. Conclusion: Chest ultrasound is considered more sensitive and specific than CXR in the diagnosis of complicated cases of acute bronchiolitis. These data suggest that a positive chest ultrasound may avoid the need to perform CXR in these patients.