A case of seminal vesicle cyst associated with ipsilateral renal dysplasia and ectopic ureter opening into seminal vesicle was experienced recently in our clinic. The case was a 14-year-old boy with the chief complaint of macrohematuria. Left kidney was not visualized in the excretory urogram and no evidence of left renal artery was seen in the abdominal aortogram. On cystoscopy the left ureteral orifice as well as the left half side of the trigone were not detected and this region appeared elevated, suggesting an extrinsic mass displacing the posterior bladder wall. The right ureteral orifice was normal. CT-scan and echogram showed the mass in the retrovesical region. The seminal vesiculogram demonstrated a seminal vesicle cyst and reflux up the left dilated ureter. After left nephroureterovesiculectomy was performed, renal dysplasia and a seminal vesicle cyst were histopathologically confirmed. The definitions in the literature for seminal vesicle cyst with maldeveloped kidney and ectopic ureter opening into the seminal vesicle with maldeveloped kidney are somewhat confusing, and they should be standardized for proper diagnosis and treatment of such cases.