The significance of low-grade prostate cancer on needle biopsy. A radical prostatectomy study of tumor grade, volume, and stage of the biopsied and multifocal tumor. Twenty-one cases showing only low Gleason grade prostate carcinoma on needle biopsy were identified. In 15 cases radical prostatectomy was performed with the entire prostate embedded for thorough evaluation of grade, volume, and stage of tumor at the needle biopsy site as well as of multifocal tumor. Eight specimens had solitary low grade and low volume tumor, with only one of these cases showing minimal capsular penetration and the others confined to the prostate. Four cases had low-grade tumor at the biopsy site, yet multifocal higher grade tumor. All of these tumor nodules were low volume and confined to the prostate. In three cases there was both low-grade and high-grade tumor in the nodule sampled by needle biopsy. In two of these cases the tumor was large and in the third it was small but mostly higher grade, with two of these cases showing capsular penetration. Although transrectal ultrasound and repeat needle biopsy would most likely have identified either the tumors' large size or high-grade component in these latter three cases, it is unlikely that these techniques would have identified the cases of multifocal higher grade tumor because of their relatively small size. Furthermore, preoperative serum prostate specific antigen levels and clinical stage failed to distinguish those cases with higher grade tumor. Because of the difficulty in identifying these areas of high-grade tumor preoperatively, it is uncertain whether expectant therapy could be recommended even for patients with very low-grade cancer on needle biopsy. 