The sterile pleural space can be invaded by organisms originating in extrathoracic and intrathoracic sites, leading to the development of a pyogenic collection referred to as empyema. From ancient times to the middle of this century, most empyemas were the result of pneumonic, traumatic, or tuberculous processes. With thoracotomy becoming a commonly performed procedure, postsurgical empyema now constitutes 20% of all cases.