We conducted ethnographic research in collaboration with a large, research‐intensive London breast cancer service in 2013–2014 so as to understand the practices and potential effects of stratified medicine. Stratified medicine is often seen as a synonym for both personalised and precision medicine but these three terms, we found, also related to distinct facets of treatment and care. Personalised medicine is the term adopted for the developing 2016 NHS England Strategy, in which breast cancer care is considered a prime example of improved biological precision and better patient outcomes. We asked how this biologically stratified medicine affected wider relations of care and treatment. We interviewed formally 33 patients and 23 of their carers, including healthcare workers; attended meetings associated with service improvements, medical decision‐making, public engagement, and scientific developments as well as following patients through waiting rooms, clinical consultations and other settings. We found that the translation of new protocols based on biological research introduced further complications into an already‐complex patient pathway. Combinations of new and historic forms of stratification had an impact on almost all patients, carers and staff, resulting in care that often felt less rather than more personal.