We present an unusual case of mucinous cystadenoma presenting with severe virilisation in a postmenopausal woman. A 71-year-old woman was referred to our outpatient endocrinology clinic because of rapidly progressive androgenic alopecia, clitoromegaly and male pattern pubic hair growth for 1 year. Her medical history was unremarkable. The serum testosterone level was 3.35 µg/L (normal range, <0.4 µg/L), and the dehydroepiandrosterone sulfate level was 267 µg/L (normal range, 100-800 µg/L). MRI of the abdomen revealed a 4×4 cm cystic ovarian mass. A bilateral salpingo-oophorectomy was performed, and histopathology showed a unilocular cystic structure with a yellowish content, compatible with mucinous cystadenoma. Postoperative testosterone levels quickly normalised (<0.4 µg/L).Rapidly developing postmenopausal hyperandrogenism easily turns into a diagnostic challenge for the clinician. Hormone-secreting neoplasms of the ovary are most commonly of sex cord stromal derivation, but atypical causes must be recognised as well. Cystadenomas are among the most common benign ovarian neoplasms and are classically considered 'non-functional' tumours. Most of these tumours are asymptomatic and found incidentally on pelvic examination or with ultrasound. To date and to the best of our knowledge, there are only five cases of mucinous adenoma causing virilisation in postmenopausal women identified in the literature. This sixth case adds strength to the link between ovarian mucinous cystadenoma and severe, rapidly progressive hyperandrogenism during menopause. In this case, surgical resection is the treatment of choice.