Recently, ultrathin bronchoscopes with a thinner external diameter, greater visual range, improved visibility, and a larger working channel have been developed. The utility of a 2.8-mm diameter ultrathin bronchoscope in diagnosing peripheral pulmonary nodules has been reported by some authors. While the feasibility of approaching peripheral pulmonary lesions is attractive, peculiar complications that have not been experienced with standard bronchoscopy may occur. We report two cases in which pneumothoraces occurred because their visceral pleuras were perforated with an ultrathin bronchoscope during the procedure. The peculiar mechanism of pneumothorax in relation to ultrathin bronchoscopy is discussed.