The survival of 78 patients with resected non-small cell lung cancer entered in prospective, randomized investigational trials is compared to that of a population-based group of control patients not included in such trials. The survival of trial patients is significantly better than that of controls (P less than 0.001). This survival advantage for trial participants is most apparent among late Stage I patients, and is observed after matching for known prognostic factors (i.e., primary tumor size, nodal status, tumor histology) and after adjusting in the analysis for age, sex, and the administration of radiation therapy. Several explanations for the improved outcome for trial patients are explored, including differences in preoperative evaluation, staging, surgical technique, placebo effects, and patient motivation. These results suggest the possibility that inclusion in these controlled cancer trials may have had an inherent advantage for all participants.