Both governments and patients' movements are increasingly making a plea in favour of the active participation of patients in biomedical research processes. One of the arguments concerns the contribution that patients could make to the relevance and quality of biomedical research based on their 'experiential knowledge'. This article reflects on the validity of patients' experiential knowledge in the context of biomedical research processes. Since a conclusive argument on the validity of patients' experiential knowledge could not be reached on the basis of theoretical reflection alone, a pragmatic approach was chosen that assessed the validity of patients' experiential knowledge in terms of its practical usefulness for biomedical research. Examples of patient participation in biomedical research were sought through literature research and more than 60 interviews with (bio)medical scientists, patients, representatives from patients' organisations, and health professionals in the Netherlands and the United Kingdom. These examples were analysed for a concrete contribution by patients to the research process. Twenty-one cases of patient participation in biomedical research were identified. After further analysis, concrete use of patients' experiential knowledge could be traced for nine of these cases. These findings suggest that patients' experiential knowledge, when translated into explicit demands, ideas, or judgements, can contribute to the relevance and quality of biomedical research. However, its deliberate use would require a more structural and interactive approach to patient participation. Since the implementation of such an approach could face various obstacles in current biomedical research practices, further research will be needed to investigate its feasibility. © 2004 Elsevier Ltd. All rights reserved.