A 17-year-old adolescent boy was referred to the urology department of our institution for hematospermia after initiation of sexual relationship. A magnetic resonance imaging scan showed giant dilation of a multicystic left seminal vesicle with left renal agenesis. These findings are typical of the Zinner syndrome. In 70%-80% of the cases when renal agenesis is found, there is an ipsilateral cystic dilation of the seminal vesicle that in some cases may be associated with testicular ectopia or absence of the bladder trigone. The ejaculatory ducts, which are formed from the mesonephric system, are abnormally developed in these cases.