IntroductionA data monitoring committee (DMC) is an independent group of experts who assess the ongoing scientific and ethical integrity of a study through periodic analyses of study data. The objective of this study was to explore the extent to which the structure, membership and deliberations of DMCs enable them to address ethical issues.MethodsWe conducted qualitative individual interviews (n=22) with DMC members from countries across Africa, the Americas, South Asia and the UK. We selected interview respondents through purposive sampling, managed data using NVivo (Release V.1.7) and analysed data thematically.ResultsAll respondents were highly experienced professionals; many (18/22) had received training in medicine and/or statistics. One respondent had academic qualifications in ethics, and four indicated that they served on DMCs as ethicists. While respondents generally felt DMCs should be required for studies that were high-risk or enrolled vulnerable populations, some were concerned about the overuse of DMCs. There were divergent views on the necessity of geographical and disciplinary representation in DMC membership, including about whether ethicists were helpful. Many respondents described a DMC member recruitment process that they felt was somewhat exclusive. While one respondent received DMC-specific training, most described learning on the job. Respondents generally agreed that study protocols and DMC charters were key guiding documents for addressing ethical issues and described DMC deliberations that often, but not always, involved consensus-building.ConclusionThis study is one of the first to consider the ethical implications of DMC structure, membership and deliberations. The potential overuse of DMCs, DMC member recruitment processes that seem somewhat insular, limited training for DMC members, and divergent approaches to deliberation may limit the capacity of DMCs for addressing ethical issues. Further research on DMC structure and processes could help enhance the ethical preparedness of DMCs.