Cyst of seminal vesicle associated with ipsilateral renal agenesis is rare. The clinical mani-festation may be asymptomatic or present with voiding or defection problems. Herein, we re-port two cases of seminal vesicle cyst with ipsilateral renal agenesis. The embryological ex-planation of this abnormality is reviewed and the differential and management dis-cussed. (J Urol R.O.C., 9:204-207,1998)