Background Service user involvement in health and social care is a key policy driver in the UK. In cancer care it is central to developing services which are effective, responsive and accessible to patients. Cancer network partnership groups are set up to enable joint working between service users and health care professionals and to drive service improvements. Aims and objectives The aim of this study was to explore the influence of the cancer network partnership groups’ service user involvement activities on cancer care. Design This was a qualitative study involving documentary analysis and in-depth case studies of a sample of partnership groups. Setting and participants Five partnership groups were purposively selected as case studies from Macmillan regions across the UK; documents were collated from a further five groups. Forty people, including core group members and key stakeholders in cancer services, were interviewed. Results and conclusions The evidence from this study suggests that cancer network partnership groups are at their most influential at ‘grass roots’ level – contributing to patient information resources, enhancing access to services, and improving care environments. While such improvements are undoubtedly important to patients, the groups’ aim is to influence strategic changes, for example in cancer care commissioning or macro-level policy decision-making. The evolution of open, participatory relationships between service users and professionals, and recognition of the value of experiential knowledge are seen as key factors in influencing cancer care. The provision of dedicated resources to strengthen service user involvement activities is also vital.