We report six cases of seminal vesicle involvement by transitional cell carcinoma of the bladder among 187 consecutive cystoprostatectomy specimens. Two of these six cases showed mucosal spread without stromal invasion (type A); the remaining four cases presented a direct extension (type B) from muscle-invasive carcinomas of the bladder. Type A involvement of the seminal vesicle was associated with a long history of superficial bladder cancer with similar mucosal spread to the prostatic ducts, acini, and ejaculatory ducts. One type A case showed extensive pagetoid spread of transitional cell carcinoma to the urethral meatus and collecting ducts of the kidney. Because the clinical significance of mucosal spread or direct invasion of seminal vesicles is not clear, pathologists and urologists need to be aware of these phenomena. More cases should be analyzed to determine further clinicopathologic implications.