RATIONALE: Intravesical migrated intrauterine devices (IUDs) have been reported to cause bladder perforation, stone formation, or malignant transition. However, such extensive intravesical benign hyperplasia caused by an extravesical migrated IUD is firstly reported. PATIENT CONCERNS: A 38-year-old woman suffered from recurrent urinary urgency and dysuria and without macroscopic hematuria for about 1 month. DIAGNOSES: Urinary ultrasound and abdominal contrast-enhanced computed tomography (CT) revealed thickening of the bladder walls. Diagnostic transurethral resection and pathology initially misdiagnosed the intravesical lesions as non-invasive urothelial carcinoma. Further diagnostic and therapeutic transurethral resections and pathology confirmed the intravesical lesions to be extensive benign hyperplasia, which was extremely likely caused by the extravesical migrated IUD. INTERVENTIONS: The intravesical lesions received therapeutic transurethral resections. Then the migrated IUD was removed by open surgery. OUTCOMES: After above treatments, the patient's lower urinary tract symptoms gradually disappeared. No recurrent lesion was found in the bladder through CT 3 months later. LESSONS: Even an extravesical migrated IUD could silently cause extensive intravesical lesions. Whether symptomatic or not, any migrated IUD including extravesical and intravesical ones should be treated seriously, if possible, removed as soon as possible.