The rate of contralateral prophylactic mastectomy (CPM) is rising rapidly, despite limited evidence about the procedure's relative benefits and harms. The objective of this study is to examine the impact of CPM on life expectancy (LE) and quality-adjusted life expectancy (QALE) in women with sporadic unilateral breast cancer. A Markov model was developed to compare 18 hypothetical cohorts of 45-year-old women with newly diagnosed unilateral, sporadic breast cancer treated with or without CPM. The probability of developing distant metastases by American Joint Committee on Cancer stage and molecular subtype was derived from British Columbia Cancer Agency data. Additional model parameters were identified from the medical literature. Sensitivity analyses were performed to examine the impact of plausible variations in key model parameters on results. CPM improved LE in all cohorts (range 0.06-0.54 years). Stage had more effect on LE than subtype (stage I mean, 0.44 years, stage III mean, 0.11 years). However, after adjusting for quality-of-life, No CPM was favored in all cohorts. Univariate sensitivity analysis demonstrated that the most influential model parameter was the post-CPM health state utility. The preferred strategy shifted from No CPM to CPM when the post-CPM utility exceeded 0.83 (base case value 0.81). PSA indicated that LE gains and QALE decreases were stable in all cohorts. The primary determinant of survival after unilateral breast cancer is stage at diagnosis. Our results suggest that routine CPM would not improve quality-adjusted survival for the majority of women with unilateral sporadic breast cancer.