A 50-year-old woman presented to the urology department with an acute history suggestive of left-sided renal colic. There were no other associated symptoms, but urine dipstick revealed non-visible haematuria. CT-KUB revealed a soft tissue mass at the left vesico-ureteric junction. Flexible cystoscopy demonstrated a mass intruding into the posterior bladder. A transurethral resection of the bladder ‘tumour’ was undertaken, and it was noted that the mass was not macroscopically consistent with transitional cell carcinoma. Histology demonstrated Müllerianosis, a rare lesion characterised by locally invasive growth of tissue originating from the Müllerian (paramesonephric) duct. The patient was seen by gynaecologist who initiated hormone treatment with an lutenising hormone - releasing hormone (LH-RH) analogue. Urological follow-up 3 months later highlighted ongoing pelvic pain but no further colicky loin pain. Repeat cystoscopy showed the mass had become smaller and the left ureter was laterally displaced. Further gynaecological input is planned if symptoms are ongoing.