About 20 or more cases of seminal vesicle cyst were reported up to nowadays, but CT findings are not yet descirbed. Seminal visicle cyst is often associated with ipsilateral renal agenssis, and the mechanism of development is based on embryogenesis. A 26 year old male was confirmed as seminal vesicle cyst in S.N.U.H. in Nov. 1981. CT of the seminal vesicle cyst revealed huge, globular and homogeneous mass with well-circumscribed smooth margin between urinary bladder and rectum. A dilated tubular structure was also found lateral to the huge cystic mass. The ipsilateral kidney was not found in any level of abdomen from the diaphragmatic dome to the symphisis pubis.