Four cases of a pelvic mass due to the development of benign hyperplasia within ectopic prostatic tissue are reported. They presented a hypoechoic homogeneous or heterogeneous pattern on transrectal ultrasonography and a high signal intensity on T2-weighted spin-echo sequences on MR. On CT the density was similar to that of prostatic tissue. No recent imaging findings of this rare entity have been described in the literature. Possible aetiologies are discussed. Even if it is a rare event, the correct diagnosis can be reached. It is important for radiologists to consider the presence of this lesion because of its benign nature.