It is widely accepted that cancer is an intersubjective experience that impacts upon the psychological well-being of people with cancer and informal carers, as well as on couple relationships. This qualitative study examined the nature and consequences of cancer on the relationship between informal carers and the person with cancer, from the perspective of Australian cancer carers. Sixty-two carers (42 women and 20 men), across a range of cancer types, stages and relationship dyads took part in semi-structured interviews. Participants reported that cancer had precipitated a change in roles and in the dynamics of the relationship, including having to take on quasi-medical tasks and decisions, neglecting self and other relationships, changes to the emotions or personality of the person with cancer, changed patterns of communication, and changes to sexuality and intimacy. The impact of the changed relationship included sadness, anger and frustration, as well as feelings of love and being closer together, resulting in relationship enhancement. Women were more likely to report changes in the person with cancer and to mourn the previous relationship, while more men reported relationship enhancement.