Background: This study examined the assessment of risk/benefit ratios for phase II cancer clinical trials by Institutional Review Board (IRB) members. Patients and methods: Semi-structured interviews were conducted with 53 IRB members from six research hospitals and specialized cancer centers in The Netherlands. Results: While the toxicity and side-effects of treatment were most often identified as risks associated with participating in a phase II trial, approximately two-thirds of IRB members also cited psychosocial and/or quality-of-life risks. Conversely, 68% of the respondents identified psychosocial benefits of trial participation, while 25% cited treatment effectiveness as a possible benefit. Between one-quarter and two-thirds of respondents indicated that trial protocols provide insufficient information regarding the likelihood, magnitude and duration of both risks and benefits. Between 15% and 34% of IRB members reported feeling less than fully competent at evaluating various aspects of phase II protocols (e.g. originality and feasibility of the study, adequacy of the methods and analysis procedures, etc.). This was particularly the case for non-physician IRB members. Few IRB members reported weighing risks and benefits in a systematic manner, but rather relied on global impressions or preferred to leave such matters to the IRB as a whole or to their patients. Conclusions: A substantial minority of IRB members believes that trial protocols provide too little information relevant to evaluating various cost/benefit and scientific issues, and feels less than fully competent in carrying out such evaluations. IRB members are more likely to identify psychosocial benefits than physical health benefits that may accrue to patients participating in phase II trials.