Background: The quality of the care relationship between a client and a professional is important in long-term care, as most clients depend on support for a lengthy period. The three largest client groups who receive long-term care in the Netherlands are older adults who are physically or mentally frail, people with mental health problems and people with intellectual disabilities. There is little clarity about how generic and variable the determinants of the quality of care relationships are across these client groups. The aim of this study is to explore and compare the determinants of the quality of care relationships in these three client groups in long-term care. Methods: This participatory study involving clients as co-researchers was held in three healthcare organizations, each providing long-term care to one client group. The research was conducted by three teams consisting of researchers and co-researchers. We interviewed clients individually and professionals in focus groups. The focus was on care relationships with professionals where there is weekly recurring contact for at least 3 months. Clients and professionals were selected using a convenience sample. The interviews were coded in open, axial and selective coding. The outcomes were compared between the client groups. Results: The study sample consisted of 30 clients and 29 professionals. Determinants were categorized into four levels: client, professional, between client and professional, and context. The findings show that the majority of the determinants apply to the care relationships within all three client groups. At the professional level, eleven generic determinants were found. Eight determinants emerged at the client level of which two were found in two client groups only. At the level between a client and a professional, six determinants were found of which one applied to mental healthcare and disability care only. Five determinants were found at the contextual level of which two were specific for two client groups. Conclusions: The study ...