Objectives: There are indications that institutional review board (IRB) members do not find it easy to assess the risks and benefits in medical experiments, although this is their principal duty. This study examined how IRB members assessed the risk/benefit ratio (RBR) of a specific phase II breast cancer clinical trial. Participants and methods: The trial was evaluated by means of a questionnaire administered to 43 members of IRBs at six academic hospitals and specialised cancer centres in the Netherlands. The questionnaire addressed: identification and estimation of inconvenience, toxicity, psychosocial distress, and benefits of trial participation to patients; identification and estimation of benefits to future patients and medical science; assessment of the trial's RBR; and assessment of its ethical acceptability. Results: Most IRB members expected trial participation to involve fairly or very serious inconvenience, fairly severe to sometimes life-threatening toxicity, and serious psychological and social consequences. Conversely, the perceived likelihood of benefits to patients was modest. Most regarded the study as important, and the balance between risks and benefits to be favourable, and believed that the protocol should be approved. The IRB members' final judgement on the trial's ethical acceptability was significantly correlated with their RBR assessment of the protocol. Conclusions: Because most patients who participate in clinical trials hope this will prolong their lives, it is suggested that patient information should better describe the anticipated benefits - for example, the likelihood of prolonging life. This would allow patients to make decisions regarding participation based on realistic expectations.