Mesonephric remnants in the prostate are an unusual mimic of adenocarcinoma with unknown incidence. This condition is considered benign, similar to its counterpart in the female genital tract, but there have only been six cases reported to date, so the histologic spectrum of this finding is uncertain. To determine the incidence and comparative histopathology of this finding, we reviewed all transurethral resections of the prostate obtained at Mayo Clinic (Rochester, MN) in 1989 to identify cases of mesonephric remnants. Among 698 prostatic transurethral resection specimens, we identified 4 cases of mesonephric remnants (0.6% incidence), all in association with nodular hyperplasia. Patients ranged in age from 66 to 82 years (mean, 72 y) and had typical urinary obstructive symptoms; follow-up was obtained in these 4 cases. Four additional consultation cases and one needle biopsy case were also included in this study. Histologically, mesonephric remnants consisted of a proliferation of benign acini arranged in lobules or showing infiltrative growth between smooth muscle bundles without stromal desmoplasia. The acini were typically round or oval, varied in size and spacing, and lined by a single layer of low cuboidal cells with scant to moderate cytoplasm and inconspicuous small nucleoli. The cells of mesonephric remnants were not reactive with antibodies to prostate-specific antigen (eight of eight cases) or with prostatic acid phosphatase (seven of seven cases); high-molecular weight cytokeratin 34betaE12 was positive in the basal cells (six of eight cases). Our results indicate that mesonephric remnants are present in <1% of transurethral resections and are rarely identified in needle biopsies. The acini are lobular or infiltrative and may be architecturally mistaken for adenocarcinoma. This cytologically innocuous finding is probably underreported and interpreted as benign prostatic acini, but this is of no apparent clinical consequence. Immunohistochemical studies with antibodies to PSA and keratin 34BE12 are helpful in separating mesonephric remnants from adenocarcinoma, similar to the case of other benign mimics.