The ball valve effect occurs when an obstructing lesion allows inspiration of air, but opposes the egression of exhaled air, causing gas trapping. The phenomenon is commonly described secondary to bronchial foreign body inhalation. However, it is less well reported in other disease processes. We report a unique case of a carcinoid tumour causing ball valving following diagnostic bronchoscopy in a young patient. The procedure caused swelling and oedema around an isolated carcinoid tumour in the left main bronchus. An inspiratory chest X-ray was normal, complicating the diagnosis. At repeat bronchoscopy the tumour was cored with LASER.