Pulmonary atelectasis in children is a pathological condition with no specific symptoms or findings that may be seen in the course of several pulmonary and thoracic diseases. Early and accurate diagnosis is essential in the treatment of pulmonary atelectasis. The first imaging method to be considered when a patient with pulmonary atelectasis is encountered is posterior-to-anterior chest X-ray. However, several pulmonary diseases can today be diagnosed and monitored using bedside ultrasonography. We describe a case of peripherally located radio-opacity in the right upper zone at posterior-to-anterior chest X-ray in a 9-year-old patient under monitoring due to asthma and presenting with cough and nasal discharge. Images compatible with atelectasis were obtained at pulmonary evaluation with bedside ultrasonography. No atelectatic areas were observed at control ultrasonography and posterior-to-anterior chest X-ray following appropriate treatment. Lung ultrasonography is an imaging technique that is especially promising in children and that may represent an alternative in the diagnosis and monitoring of pulmonary atelectasis.