Twenty-five patients with localized prostate cancer underwent seminal vesicle biopsies before radical prostatectomy. A transrectal probe of 7 MHz, a 18-gauge needle and a biopsy gun were used. The preoperative biopsy established the absence of seminal vesicle invasion in 89% of cases. When the seminal vesicles are positive at biopsy, capsular penetration is observed in 100% of the cases and lymph node positivity in 50%. When seminal vesicles are negative at biopsy and the prostate-specific antigen level is less than 20 ng/ml (n less than 2.5), capsular penetration of greater than 1 cm is absent in 100% of cases and lymph nodes are positive in only 7% of cases. Biopsy of the seminal vesicle, as an outpatient procedure, improves the preoperative staging of prostate cancer before radical prostatectomy: negative biopsies are good predictors of the absence of lymph node invasion.