It has often been argued that if a clinician cannot decide which of two treatments to offer, a trial may be ethical, but it is unethical if she/he has a preference. Since individual clinicians usually have a preference, most trials could be judged unethical according to this line of argument. A recent important article in the New England Journal of Medicine argued that individual preferences are not as important as the collective uncertainty of informed clinicians. If clinicians are equally divided, (...) there is a state of collective equipoise and a trial is ethical. However, clinicians will seldom be exactly equally divided. We conducted an ethometric study to find out how much collective equipoise can be disturbed before the potential subjects in a trial think that it is unethical. Half of our subjects perceived a trial as unethical when equipoise was disturbed beyond 70:30. In other words, when 70 per cent of experts favour one treatment, 50 per cent of subjects would prefer that treatment to be administered rather than subjected to critical assessment. When equipoise is disturbed beyond 80:20, less than 3 per cent of subjects would consider human trials morally justifiable. (shrink)