A 5-month-old boy was diagnosed as having complete atelectasis of the right lung due to RS virus infection at the age of 1 month. Conventional respiratory physical therapy, inhalation therapy and mechanical ventilation through an endotracheal tube failed to re-expand the right lung, while the left lung gradually became overinflated. We therefore tried differential lung ventilation by using a combination of a laryngeal mask airway and an extra long endotracheal tube (ID, 3.5 mm; length, 280 mm; Portex Pediatric Tracheal Tube, Extra length; SIMS Portex Co., Ltd. UK). Following induction of anesthesia, a laryngeal mask airway #2 was inserted. Then, an extra long endotracheal tube was inserted through the laryngeal mask airway and was guided to the right main bronchus with the aid of a fiberscope. The right lung was selectively lavaged and inflated with high pressure while ventilation was maintained through the laryngeal mask airway. The SpO2 value was maintained at more than 95% throughout the procedure despite some leakage from the ventilation system. The case demonstrates that differential lung ventilation by use of a combination of a laryngeal mask airway and extra long endotracheal tube is useful for the treatment of a pediatric patient with severe atelectasis.