Müllerian duct cysts result from an abnormality in regression of the Müllerian system. They may occasionally give rise to symptoms. We report an unusual case of acute urinary retention in an old man caused by a giant Müllerian duct cyst.A 77-year-old man presented with of acute urinary retention. After bladder drainage, digital rectal examination found a large soft supraprostatic mass. Transrectal ultrasound and computed tomography scans revealed a large multilocular retrovesical cystic mass. The patient underwent open surgical resection of the cyst. Histologically, the cystic lesion was lined with stratified cubocolumnar cells, consistent with a Müllerian duct cyst.Acute urinary retention in the elderly is not always related to prostatic diseases. Other causes, even congenital ones, may be involved