Yes / Service user involvement is embedded in the United Kingdom's National Health Service, but knowledge about the impact of involvement on service users, such as the benefits and challenges of involvement, is scant. Our research addresses this gap. To explore the personal impact of involvement on the lives of service users affected by cancer. DESIGN: We conducted eight focus groups with user groups supplemented by nine face-to-face interviews with involved individuals active at a local, regional and national level. Thematic analysis was conducted both independently and collectively. SETTING AND PARTICIPANTS: Sixty-four participants, engaged in involvement activities in cancer services, palliative care and research, were recruited across Great Britain. RESULTS: We identified three main themes: (i) 'Expectations and motivations for involvement'- the desire to improve services and the need for user groups to have a clear purpose, (ii) 'Positive aspects of involvement'- support provided by user groups and assistance to live well with cancer and (iii) 'Challenging aspects of involvement'- insensitivities and undervaluing of involvement by staff. CONCLUSIONS: This study identified that involvement has the capacity to produce varied and significant personal impacts for involved people. Involvement can be planned and implemented in ways that increase these impacts and that mediates challenges for those involved. Key aspects to increase positive impact for service users include the value service providers attach to involvement activities, the centrality with which involvement is embedded in providers' activities, and the capacity of involvement to influence policy, planning, service delivery, research and/or practice.