Adjuvant treatment with aromatase inhibitors improves outcomes in postmenopausal women with hormone-sensitive early breast cancer; however, they should not be used in premenopausal women. Menopausal status is the most important factor in the choice of the hormonal treatment. There is no direct correlation between amenorrhea and ovarian function, as even the patients with amenorrhea may present with premenopausal plasma estradiol levels. The evaluation of hormonal status becomes more complicated in patients taking tamoxifen, which might lead to further increase of plasma estradiol levels. Therefore, its evaluation before and during the treatment with aromatase inhibitors is clinically important. There is a considerable caution needed when indicating aromatase inhibitors in patients with menopause caused by previous adjuvant chemotherapy, while recovery of ovarian function may appear after a certain period. This could take from 4 to 59 months (12 months on average) and it might not be accompanied by menses. This happens typically in women younger than 40 years, who should, therefore, not be treated by aromatase inhibitors alone. This supports the notion that monitoring of plasma estradiol levels is crucial in women from 40 to 50 years of age, especially before the start of aromatase inhibitors treatment.