Abstract Background There is growing recognition of the importance of patient and public stakeholder involvement (PPI) in patient preference research. However, limited evidence exists regarding the impact, barriers and enablers of PPI in preference studies. The Innovative Medicines Initiative (IMI)-PREFER project conducted a series of preference case studies which incorporated PPI. Objective To describe: (1) how PPI was operationalized in the PREFER case studies, (2) the impact of PPI, and (3) factors that served to impede and facilitate PPI. Methods We reviewed the PREFER final study reports to determine how patient partners were involved. We conducted a thematic framework analysis to characterize the impact of PPI and then administered a questionnaire to the PREFER study leads to identify barriers and facilitators to effective PPI. Results Eight PREFER case studies involved patients as research partners. Patient partners were involved in activities spanning all phases of the patient preference research process, including in study design, conduct and dissemination. However, the type and degree of patient partner involvement varied considerably. Positive impacts of PPI included improvements in the: (1) quality of the research and research process; (2) patient partner empowerment; (3) study transparency and dissemination of results; (4) research ethics, and (5) trust and respect between the research team and the patient community. Of the 13 barriers identified, the 3 most frequently reported were inadequate resources, insufficient time to fully involve patient partners, and uncertainty regarding how to operationalize the role of ‘patient partner. Among the 12 facilitators identified, the two most frequently cited were (1) having a clearly stated purpose for involving patients as research partners; and (2) having multiple patient partners involved in the study. Conclusion PPI had many positive impacts on the PREFER studies. Preference study leads with prior PPI experience reported a greater number of positive impacts than those with no such experience. In light of the numerous barriers identified, multi-faceted implementation strategies should be considered to support adoption, integration and sustainment of PPI within preference research. Additional case studies of patient partner involvement in preference research are needed as well to inform best practices in this area.