Broncho-pleural fistula is a relatively rare but life-threatening complication of pneumonectomy. Early detection of this complication is crucial for optimal treatment. We describe a simple technique to detect postpneumonectomy broncho-pleural fistula by measuring concentrations of O2 and N2O in the pneumonectomy cavity at baseline and after allowing patients to breath gas mixtures enriched with O2, N2O, or both. The technique was used in 22 patients. In 20 control patients the test was carried out 48 hours after pneumonectomy. Both the O2 and the N2O test were negative in all cases. However, in 2 patients with broncho-pleural fistula both the O2 and the N2O tests were positive.