Congenital cystic disease of the seminal vesicle. Thirteen cases of congenital seminal vesicle cysts with pathologic correlation were diagnosed between 1970 and 1988. Twelve of the 13 patients had ipsilateral renal anomalies. Intravenous urography, performed in 11 of the 13 patients, demonstrated associated renal anomalies. Computed tomography, performed in nine of the 13 patients, demonstrated associated renal anomalies and displayed the cystic seminal vesicles. Transabdominal or endorectal ultrasonography, performed in eight patients, allowed characterization of the seminal vesicle masses as cystic. Magnetic resonance imaging, performed in three of the 13 patients, accurately demonstrated dilated ejaculatory ducts into which ectopic ureters inserted. The fluid in the seminal vesicle cysts had an increased signal intensity on T1- and T2-weighted sequences. Seminal vesiculographic study demonstrated anomalous communications with the seminal tract. Cystic disease of the seminal vesicles can be either congenital or acquired; congenital cysts are associated with anomalies of the ipsilateral mesonephric duct. 