Background: This article charts the relationships between the model of evidence-based practice (EBP), healthcare markets where providers are increasingly competing through the adoption of EBP-certified interventions, and the cultivation of experiential knowledge within self-help and mutual aid groups (MAGs). After 35 years of neoliberal reform, service user involvement in research, service provision and evaluation, and patient-centered care has been operationalised in increasingly measurable ways. In seeking to value and incorporate service user experiences, current models of EBP do not unpack the heterogeneity within experiential knowledge. Aims: This article explores a more meaningful use of experiential knowledge than the cursory and tokenistic treatment it is often given. Objectives: Propose, illustrate and theorise the concept of 'deep experiential knowledge' (DEK). Identify ways that the acknowledgement of DEK are useful in healthcare policy, governance and the clinical encounter. Methods: Drawing upon case study vignettes, we analyse MAGs as epistemic communities of problem solvers. Findings Deep experiential knowledge is a robust and collective form of knowledge, generated over time in the long-term members ('old-timers') and collective knowledge of MAGs. Five characteristics of deep experiential knowledge are proposed. Discussion: By rendering DEK amenable to the logic of EBP, we outline potential benefits of foregrounding DEK in the conduct of healthcare research, policy and governance, and the clinical encounter. Conclusions: DEK constitutes an authority that distinguishes different degrees of experiential knowledge of healthcare problems. Attending to DEK helps untangle some of the challenges posed by EBP for and to successful service user involvement.