Background There is an increasing international interest in patient and public involvement (PPI) in research, yet relatively little robust evidence exists about its impact on health and social care research. Objective To identify the impact of patient and public involvement on health and social care research. Design A systematic search of electronic databases and health libraries was undertaken from 1995 to 2009. Data were extracted and quality assessed utilizing the guidelines of the NHS Centre for Reviews and Dissemination 2009 and the Critical Appraisal Skills Programme (CASP). Grey literature was assessed using the Dixon‐Woods et al. (2005) checklist. Inclusion criteria All study types that reported the impact PPI had on the health and/or social care research study. Main results A total of 66 studies reporting the impact of PPI on health and social care research were included. The positive impacts identified enhanced the quality and appropriateness of research. Impacts were reported for all stages of research, including the development of user‐focused research objectives, development of user‐relevant research questions, development of user‐friendly information, questionnaires and interview schedules, more appropriate recruitment strategies for studies, consumer‐focused interpretation of data and enhanced implementation and dissemination of study results. Some challenging impacts were also identified. Conclusion This study provides the first international evidence of PPI impact that has emerged at all key stages of the research process. However, much of the evidence base concerning impact remains weak and needs significant enhancement in the next decade.