Almost a third of the 1.3 million women diagnosed with breast cancer each year are premenopausal or perimenopausal, and about three quarters have endocrine sensitive disease.1 2 3 Many of these women are treated with cytotoxic chemotherapy. This treatment significantly improves survival outcomes,4 5 but it is also associated with an increased risk of temporary or permanent amenorrhoea. In recent years, aromatase inhibitor therapy has become a global standard of care for postmenopausal women with receptor positive disease.6 However, an increasing number of women who are premenopausal or perimenopausal at the time of diagnosis and whose menses stop with adjuvant chemotherapy are also being given aromatase inhibitors as monotherapy (that is, without concomitant ovarian suppression or ablation), even though their use as single agents is absolutely contraindicated in premenopausal or perimenopausal women. Therefore, accurate determination of menopausal status is vital in the effective endocrine management of women with breast cancer. We present a case of incorrect assessment of menopausal status and discuss its effects.