In premenopausal women treated for breast cancer, endocrine therapy combining an aromatase inhibitor (AI) and a gonadotropin-releasing hormone (GnRH) agonist (GA) for ovarian suppression may be indicated in high-risk or in metastatic cancer. AIs are effective in premenopausal women only when ovarian estrogen production is suppressed, a state achievable through the use of GAs. However, a complete suppression sometimes proves elusive. We report here three cases of ovarian suppression failure in premenopausal breast cancer patients who received adjuvant AI+GA. Frequency of GA administration, BMI, and young age could affect gonadotropin suppression and may be implied in these failures. Clinical monitoring of these patients is advisable, and hormone assays and pelvic ultrasound should be performed in case of symptoms of estrogen activity.