PURPOSE To report the case of a patient who underwent right thoracoscopic pleurectomy with lung exclusion and developed contralateral (left) pneumothorax with resulting life-threatening alteration of the respiratory and cardiovascular functions. CLINICAL FEATURES A 28-yr-old male was admitted to the intensive care unit for a well tolerated, second episode of spontaneous right pneumothorax and scheduled for right thoracoscopic pleurectomy. Anesthesia was induced and maintained with sufentanil and propofol. A double lumen endotracheal tube (ETT) was inserted, its correct positioning checked clinically and by fiberoptic bronchoscopy and the patient was placed in the left decubitus position. Approximately one hour into the procedure, during the second period of right pulmonary exclusion, SpO2 values decreased within two minutes to 78%. End tidal capnography (EtCO2) values decreased to 6-8 mmHg within seconds and peak airway pressure increased to values between 50 and 60 cm H2O. Severe cyanosis, sinus bradycardia and arterial hypotension developed. The surgical procedure was stopped, propofol administration discontinued, bipulmonary ventilation reinstituted and the patient placed in the supine position which restored hemodynamic and respiratory function. Inspection and auscultation were consistent with tension left pneumothorax which was evacuated. CONCLUSION Pneumothorax of the ventilated lung during one lung ventilation for thoracoscopic procedures must be diagnosed quickly. Reinstitution of bipulmonary ventilation should probably be the first therapeutic attitude.