In a group of 22 patients undergoing thoracotomy we compared two techniques of ventilatory assistance to the nondependent lung during single lung ventilation. We simultaneously administered a 0.5% FiO2 to the dependent lung. We used a CPAP system with continuous O2 flow limited by an underwater valve at a pressure of +5 cmH2O. We performed 33 ventilatory assistances: in 15 cases to the nondependent lung (CPAP group) and in 18 patients to the lower lobe of the nondependent lung (lobar CPAP group). Evaluation of both techniques was performed by means of arterial blood gas measurement and the mean values were compared using the student's t test. During single lung ventilation the PaO2 in CPAP group increased from 85.86 +/- 22.28 mmHg to 155.52 +/- 59.54 mmHg (p less than 0.001) and in the lobar CPAP series it increased from 88.75 +/- 24.34 mmHg to 122.36 +/- 43.21 mmHg (p less than 0.01). In 11 out of the 22 patients we firstly applied the lobar CPAP and thereafter the CPAP to the whole lung in order to compare the efficacy of both techniques in the same patient. The PaO2 during single lung ventilation was 86.9 +/- 22.7 mmHg and it increased to 111.1 +/- 37.9 mmHg after lobar CPAP (p less than 0.01) ant to 163.3 +/- 64 mmHg after total lung CPAP ventilation (p less than 0.001). Our results confirm the usefulness of both techniques and they indicate that CPAP to the whole nondependent lung is the most effective.