Aggressive angiomyxoma (AA) is a rare mesenchymal tumour which mainly arises in the soft tissue of the pelvis and perineum in women of reproductive age. AA usually shows an aggressive behaviour, with a high rate of incomplete surgical excision and post-surgical recurrence. Most cases of AA exhibit oestrogen and/or progesterone receptors; in these cases, maintenance of a hypo-oestrogenic state can be helpful in the medical management of this tumour. We describe a case of spontaneous reduction in size of an AA during a 6-month period of hormonal replacement therapy withdrawal, assessed by MRI using a 3T magnet.