We present a large hydatid cyst located midline and posterior to prostatic gland and seminal vesicles in an asymptomatic patient. Computed tomography (CT) scan revealed a large retrovesical mass and the transrectal ultrasonography idetified a degenerated hydatid cyst with "ball of wool'' appearance. Although extremely rare, hydatid disease should be kept in mind in the differential diagnosis of pelvic particularly retrovesical midline masses.