Background & Objective: Involving patients in scientific research has been shown to improve the relevance of the research, as well as its quality and applicability. Harteraad, the Dutch patient organization for people with cardiovascular diseases, has a Committee of Experienced Experts (patients) advising researchers on the content of grant proposals prior to submission. Until now, the impact of the committee's advice was unknown. This study, initiated by Harteraad, aimed to evaluate the impact of the provided advice on the content of grant proposals and investigate how to strengthen this impact. Methods: Fourteen grant proposals both prior to and after receiving the committee's advice were compared in order to analyse how the advice had been incorporated into the final proposal. Subsequently, 10 researchers who received the committee's advice were interviewed. Moreover, a focus group discussion was conducted with five committee members. Results: Document analysis showed that almost 40% of the advice was incorporated in the final grant proposals. Researchers made several changes to their proposals, such as increasing the extent of patient involvement throughout the research, use of simpler language, and/or adding information on the consequences of an intervention for patients. Advice requiring fundamental changes in the research design was most often not incorporated. This finding was confirmed by the interviewees, although some stressed to use the committee's advice later on during the execution of the research. According to the interviewees and members of the committee, the impact of the committee's advice could be strengthened in several ways, including 1) improving training/education for researchers and the committee, 2) organizing dialogues between patients and researchers, 3) aligning perspectives between funding bodies and patient organizations on what is expected from researchers, 4) making it obligatory for the researchers to clarify how the patient's advice was incorporated, and 5) fostering researchers' internal motivation for involvement. Committee members have contributed to implementing these recommendations. Conclusion: The committee's advice has considerable impact on the content of grant proposals. However, effort is required to increase the value that is currently attributed to patient involvement, and to support researchers in the required organizational and cultural changes to meaningfully involve patients in research.