The problem of how to improve postoperative pulmonary function after upper abdominal surgery was investigated in a randomized study involving 64 patients who were subjected to various treatment regimens designed to increase airway pressure. Intraoperative ventilation was carried out with either zero or positive end exspiratory pressure, and postoperatively either CPAP or a nasal oxygen catheter were applied. The following groups were formed: 1. ZEEP /O2-catheter; 2. PEEP/O2-catheter; 3. ZEEP /CPAP; 4. PEEP/CPAP. The typical reduction in vital capacity occurred postoperatively, the lowest value being recorded on the 2nd postoperative day, an alteration in respiratory pattern with reduced tidal volume and increased respiratory rate together with an initially low, later normal alveolar ventilation, and an initial hypoxaemia which was at first associated with a moderate hypercapnia, on the second postoperative day with a normocapnia, were observed. At no point in time could a difference be found between the 4 groups, no measurable improvement in respiratory function being found as a result of the treatment given. Intra- and post-operative increase in airway pressure was however found to be associated with a reduction in the incidence of post-operative pulmonary complications.