We evaluated 64 patients with clinically localized prostate cancer (on the basis of rectal examination, serum acid phosphatase, bone scan and pelvic computerized tomography scan) by transrectal sonography before radical prostatectomy. Of the 48 patients with histologically proved localized prostate cancer sonography overstaged the disease in 5 (10%) and correctly staged it in 43 (90%). All overstaged cancer patients were scanned after either prostatic biopsy (4) or transurethral prostatectomy (1) established the diagnosis of prostate cancer. Of the 16 patients with histologically proved, locally advanced prostate cancer (that is extracapsular extension and/or seminal vesicle invasion) sonography understaged the disease in 10 (62%) and correctly staged the disease in 6 (38%). These data suggest that sonography is associated with considerable staging errors when used to evaluate men with clinically localized prostate cancer.