Background: Rates of both unilateral (UM) and contralateral prophylactic mastectomy (CPM) for early stage breast cancer (ESBC) have been increasing since 2003. More extensive surgery is not a benign procedure without the risk of complications. Studies suggest that the increase is due to women choosing UM and CPM; we do not know what factors are influencing the choice for more extensive surgery. Methods: We conducted a qualitative study using grounded theory to identify factors for the choice of mastectomy. Purposive sampling was used to identify women across the Toronto Area (Ontario, Canada), who were suitable candidates for breast conserving surgery (BCS) but underwent UM or CPM. Data were collected through semi-structured interviews. Constant comparative analysis identified key concepts and themes. Results: Data saturation was achieved after 29 in-person interviews. 12 interviewees were treated at academic cancer centers, 6 at an academic non-cancer center and 11 at community centers. 15 women underwent UM; 14 underwent UM+CPM. Median age was 55. ‘Taking control of cancer’ was the dominant theme. Fear of breast cancerwas expressed at diagnosisand remains throughout decision making. Fear translates into the overestimated risk of local recurrenceand contralateral cancer. Despite discussion of the equivalence of BCS and UM, patients chose UM due to fear of recurrence and misperceived survival advantage. Similarly, patients chose CPM to eliminate the risk of contralateral cancer and misperceived survival advantage. Women were actively trying to Control Outcomes, as more surgery was seen as greater control. Conclusions: Women seeking UM and CPM for treatment of their early stage breast cancer manage their fear of cancer by undergoing more extensive surgery which in turn drives mastectomy rates. It is important to understand this process so that we may improve our ability to discuss issues of importance to women and facilitate informed decision-making.