Four hundred and sixty-one doctors who treat lung cancer in Canada and the United States answered a questionnaire in which they were asked how they would wish to be managed if they developed non-small cell lung cancer (NSCLC). There was no evidence of a consensus as to preferred treatment in either of two clinical situations described. Personal treatment preferences were significantly influenced by specialist training and each discipline showed a preference for its own modality of treatment. The personal treatment preferences of American and Canadian doctors differed significantly. In the United States, the role of surgery in NSCLC with extensive mediastinal disease was controversial, whereas in Canada, the major controversy was whether any active treatment was desirable in this situation if symptoms were absent. The role of chemotherapy in the treatment of NSCLC with painful bone metastases was controversial in the United States, but the vast majority of Canadian doctors would not wish any form of chemotherapy in this situation. Respondents were also asked what treatment they usually recommended for patients with NSCLC in the two situations described. Almost all these doctors recommended for their patients exactly the same treatment which they would choose for themselves. It was concluded that the personal treatment preferences of doctors are an important factor in determining how patients with NSCLC are treated. Doctors were also asked (a) if they would be willing to participate as patient-subjects in a number of clinical trials for which they would be eligible if they developed NSCLC, and (b) if they would be willing to ask their patients to participate in the same trials. There were significant differences in the perceived acceptability of the trials studied, but in each case a higher proportion of doctors would be willing to ask their patients to participate than would be prepared to consent themselves.