OBJECTIVES: To determine physicians' attitudes and reactions to their patients' use of alternative cancer therapies, factors that affect these reactions and physicians' views of how the use of such therapies affects the physician-patient relationship. DESIGN: Qualitative study involving in-depth semistructured interviews. SETTING: Toronto. PARTICIPANTS: Nineteen oncologists and 35 general practitioners (GPs) were selected by means of purposive sampling; 18 oncologists and 12 GPs agreed to participate. OUTCOME MEASURES: Attitudes and reactions to patients' use of alternative cancer therapies; factors affecting physicians' reactions to such use; and physicians' views of how the use of such therapies affects the physician-patient relationship. RESULTS: Many physicians perceived themselves to be unfamiliar with available alternative cancer therapies and indicated that their main sources of information were their patients and the lay press. Although most of the physicians viewed the efficacy of such therapies as scientifically unproven, they would respect their patients' decision to use them and encourage them to continue with standard treatment. Factors found to influence the physicians' reactions included the prognosis with standard treatments, the exclusivity of the use of alternative therapies and whether the alternative therapies were harmful. Although many of the participants felt that a patient's use of alternative cancer therapies did not affect the physician-patient relationship, a few indicated that it did cause some tension. CONCLUSION: Because many physicians lack information on alternative cancer therapies and most of these therapies have not been scientifically proven, physicians' attitudes and reactions to their use by patients are influenced to a greater degree by the efficacy or inefficacy of standard treatment and the invasiveness of the alternative therapy than by the efficacy of the alternative therapy used.