A 52-yr-old man presented with severe obstructive urinary symptoms. Ten years earlier, a digital rectal examination disclosed a small mass above the prostate, and a computed tomography (CT) scan showed a 3.5-cm cystic tumor of the right seminal vesicle. He had been followed conservatively elsewhere. Reevaluation of the mass with a CT scan and magnetic resonance imaging showed that the mass had grown to a maximal diameter of 14 cm. A transabdominal needle biopsy revealed benign fibromuscular tissue. The tumor was then resected by an open transvesical approach. Pathology was consistent with a benign seminal vesicle cystadenoma. The natural history, pathology, and surgical approach are described.