Objective--To investigate the effects of deep breathing performed on the second postoperative day after coronary artery bypass graft surgery. Design--The immediate effects of 30 deep breaths performed without a mechanical device (n = 21), with a blow bottle device (n = 20) and with an inspiratory resistance-positive expiratory pressure mask (n = 20) were studied. Spiral computed tomography and arterial blood gas analyses were performed immediately before and after the intervention. Results--Deep breathing caused a significant decrease in atelectatic area from 12.3 +/- 7.3% to 10.2 +/- 6.7% (p < 0.0001) of total lung area 1 cm above the diaphragm and from 3.9 +/- 3.5% to 3.3 +/- 3.1% (p < 0.05) 5 cm above the diaphragm. No difference between the breathing techniques was found. The aerated lung area increased by 5% (p < 0.001). The PaO (2) increased by 0.2 kPa (p < 0.05), while PaCO (2) was unchanged in the three groups. Conclusion--A significant decrease of atelectatic area, increase in aerated lung area and a small increase in PaO (2) were found after performance of 30 deep breaths. No difference between the three breathing techniques was found.