Aims  (i) To describe patient and public involvement (PPI) in a network promoting research in dementia and neurodegenerative diseases, in terms of activity at the different stages of the research cycle and within the different levels of the research network. (ii) To use case studies to try and answer the question: what benefits (if any) does PPI in research bring to the research process?Background  PPI in health research is a central part of government policy, but the evidence base underpinning it needs strengthening. PPI allows exploration of feasibility, acceptability and relevance of hypotheses, assists in the precise definition of research questions and increases accrual to studies. However, the measurement of outcomes is methodologically difficult, because the impact of lay researchers may occur through team interactions and be difficult to untangle from the efforts of professional researchers. Opportunities for PPI in rapidly progressive diseases may be limited, and involvement of people with marked cognitive impairment is particularly challenging.Design  (i) Description of PPI within the DeNDRoN network. (ii) Case studies of three research projects which asked for extra help from centrally organized PPI.Results  PPI in research projects on the DeNDRoN portfolio may function at different levels, occurring at project, local research network and national level. Case studies of three research projects show different roles for PPI in research and different functions for centrally organized PPI, including contribution to remedial action in studies that are not recruiting to target, solving problems because of the complexity and sensitivity of the research topic, and linking researchers to PPI resources.Discussion  The case studies suggest that centrally organized PPI can have ‘diagnostic’ and remedial functions in studies that are struggling to recruit and serve as reinforcement for study‐level PPI in the complex and sensitive research topics that are typical in neurodegenerative diseases research. PPI may be actively sought by researchers, but the infrastructure of PPI is not yet so widespread in the research community that lay researchers are easy to find; a centrally organized PPI resource can assist in this situation.