Mature ovarian dermoid cysts are common lesions, accounting for up to 10–25% of all ovarian neoplasms. Uncomplicated dermoid cysts are often asymptomatic and are relatively easy to diagnose on imaging and to treat. Symptoms develop once complications set in and these may cause diagnostic dilemmas. Torsion (16%) is the most common complication, while rupture, suppuration and malignant transformation are relatively uncommon. Of all these complications, spontaneous rupture into the urinary bladder is least common. The diagnosis of this condition has been through the use of cystoscopy or laparotomy in all cases reported so far. We report a case of a 30-year-old patient with pyuria and dysuria, where ultrasound examination clearly demonstrated an ovarian dermoid cyst invading the urinary bladder. A clear-cut imaging diagnosis helped to allow planning of surgery in advance and a mucosa-sparing partial bladder resection could be performed. Copyright © 2009 ISUOG. Published by John Wiley & Sons, Ltd.