The 12-min walking distance was measured as part of the pre-operative evaluation of 30 patients with carcinoma of the bronchus consecutively admitted to hospital for lung resection. Of the 22 who had a resection, 7 suffered post-operative ventilatory complications. There was no significant difference in distance walked, both in absolute terms and when standardised for effort, between these patients and the 15 who did not suffer post-operative complications. The empirically derived level of maximum separation between the complication and non-complication groups was in the region of 1,100 m. This approach also failed to provide significant separation between the two groups. Significant separation was achieved by discriminant analysis of simple pulmonary function tests but this separation was not enhanced by entry of values from the 12-min walk into the analysis. These results suggest that there is little to be gained from adding exercise testing to routine pulmonary function tests in pre-operative assessment before lung resection, and that the use of exercise tolerance as a final arbiter of fitness for operation should be viewed with caution.