OBJECTIVE: Phase I oncology trials have raised concerns that patients' 'unrealistic' optimism could compromise the validity of informed consent, and that patients often participate in trials to conform to physicians' or family members' recommendations. We aimed to determine whether patients or families-given the same information of risk-benefit profile-are more likely to participate in Phase I trials than their physicians and whether people in family or physician situations are more likely to recommend trial participation to patients than they would want for themselves as patients. METHODS: We conducted a hypothetical vignette study with a patient-caregiver-oncologist. Three groups-725 patient-caregiver pairs recruited by 134 oncologists-were asked to assume three different roles as patients, caregivers and physicians and provided a scenario of a hypothetical patient with treatment-resistant cancer. They were asked questions regarding their intention to participate in or to recommend a Phase I clinical trial. RESULTS: Acceptance rates of the trial were as follows: (a) in the patients' role: patients (54.1), caregivers (62.3) and physicians (63.4%): (b) in the caregivers' role: 55.6, 64.7 and 70.9%: (c) in the physicians' role: 66.1, 70.8 and 76.1%. Patients or caregivers were not more positive to the trial than physicians. All three groups showed more positive attitudes toward the clinical trial when they assumed the role of caregiver or physician than that of patient. CONCLUSIONS: Patients and caregivers seem to make as reasonable decisions as physicians: patients seem to take family members' or physicians' recommendation as their legitimate roles rather than as undue pressure.