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Purpose: To determine if women with breast cancer that undergo fertility preservation (FP), with or without hormonal stimulation, present with an increased risk of breast cancer recurrence.Methods: A matched cohort study on women with breast cancer attempting to ensure FP in Stockholm from 1999 to 2013 [exposed women (n = 188), age-matched unexposed controls (n = 378)] was designed using the Stockholm regional data from the Swedish National Breast Cancer Quality Register. Breast cancer relapse rates [incidence rate ratio (IRR)] and 95% confidence interval (CI) were estimated using Cox regression and adjusted for potential confounding factors. Completeness of the registry at the time of the study was close to 99%.Results: Most women attempted FP by hormonal stimulation treatment (n = 148, 79%) with the objective of freezing their eggs or embryos. A smaller group elected FP methods without hormone stimulation (n = 40, 21%). Women who received hormone stimulation did not present with a higher relapse rate than unexposed control women in a model adjusted for age and calendar period of diagnosis (IRR 0.59, 95% CI 0.34–1.04). The results remained virtually unchanged after adjustment for tumor size, estrogen receptor status, affected lymph nodes, and chemotherapy treatment (IRR 0.66, 95% CI 0.37–1.17).Conclusion: Evidence was not found that fertility preservation, with or without hormonal stimulation, was associated with an increased risk of breast cancer recurrence. The high coverage rate of this population-based study supports the safe practice of fertility preservation in young women with breast cancer. [ABSTRACT FROM AUTHOR]