Collecting and documenting subjective prior beliefs from knowledgeable clinicians about the potential results of a clinical trial has many advantages. Two large trials of prophylactic treatments in an HIV-positive population are used as examples. The trials recruited patients of primary care physicians and compared treatments which were in use in clinical practice. Opinions about these trials were elicited from 58 practising HIV clinicians. It is shown how the documented opinions can be used to augment the monitoring process; the prior opinions are updated with interim data using approximate Bayesian methods to give posterior opinions incorporating interim results. These posterior opinions can be used by the monitoring board to anticipate the clinicians' reaction to the results. Eliciting prior beliefs is also ethically important for documenting the nature of the uncertainty or equipoise. Important information is provided for the informed consent process and Institutional Review Board (IRB).