Intraoperative investigations after axillary thoracotomy in 28 patients with spontaneous pneumothorax shows true ruptures of subpleural bullae in 25% of the cases. In 75% structural pleural changes in the top of the lung are responsible for porosity and air leakage of the wall of the bullae. Pleural porosity causes a prolonged treatment of this illness. Appreciation of the phenomenon of "pleural porosity" is the key for understanding several questions regarding the diagnosis and treatment of spontaneous pneumothorax that had remained unanswered to date.