BACKGROUND: Peer review is the most widely used method for evaluating grant applications in clinical research. Criticisms of peer review include lack of equity, suspicion of biases, and conflicts of interest (CoI). CoIs raise questions of fairness, transparency, and trust in grant allocation. Few observational studies have assessed these issues. We report the results of a qualitative study on reviewers' and applicants' perceptions and experiences of CoIs in reviews of French academic grant applications. METHODOLOGY AND PRINCIPAL FINDINGS: We designed a qualitative study using semi-structured interviews and direct observation. We asked members of assessment panels, external reviewers, and applicants to participate in semi-structured interviews. Two independent researchers conducted in-depth reviews and line-by-line coding of all transcribed interviews, which were also subjected to Tropes® software text analysis, to detect and qualify themes associated with CoIs. Most participants (73/98) spontaneously reported that non-financial CoIs predominated over financial CoIs. Non-financial CoIs mainly involved rivalry among disciplines, cronyism, and geographic and academic biases. However, none of the participants challenged the validity of peer review. Reviewers who felt they might be affected by CoIs said they reacted in a variety of ways: routine refusal to review, routine attempt to conduct an impartial review, or decision on a case-by-case basis. Multiple means of managing non-financial CoIs were suggested, including increased transparency throughout the review process, with public disclosure of non-financial CoIs, and careful selection of independent reviewers, including foreign experts and methodologists. CONCLUSIONS: Our study underscores the importance of considering non-financial CoIs when reviewing research grant applications, in addition to financial CoIs. Specific measures are needed to prevent a negative impact of non-financial CoIs on the fairness of resource allocation. Whether and how public disclosure of non-financial CoIs should be accomplished remains debatable.