Morphometric reconstructions of 68 consecutive radical prostatectomies were analyzed for cancer volume, extent of complete capsular penetration, microscopic seminal vesicle and lymph node invasion, and histological differentiation, all of which were strongly interrelated. At less than 3.0 cc cancer volume, only 6 of 34 prostates (18 per cent) showed capsular penetration compared to 27 of 34 (79 per cent) with tumors of greater than 3.0 cc. Seminal vesicle invasion occurred once in 34 tumors of less than 3.0 cc and 15 times in those greater than 3.0 cc. All 6 patients with metastases to lymph nodes, 2 with early postoperative development of bone metastases and 4 of 5 with reappearance of detectable prostate specific antigen postoperatively had cancer volumes of greater than 4.0 cc. Correlation of digital rectal examination with cancer volume showed that of 39 palpable nodules in prostates with a cancer volume of less than 4.0 cc 30 (77 per cent) occupied 50 per cent or less of the length of 1 lobe (clinical stage B1 in our classification). Of 22 palpable lesions in tumors of greater than 4.0 cc 21 (95 per cent) exceeded 50 per cent of 1 lobe in the longitudinal extension (stage B2) or they represented bilaterally palpable disease (stage B3). Capsular penetration into the periprostatic fat occurred most commonly in the dorsolateral area of the neurovascular bundle, including 10 of 12 tumors less than 4.0 cc in volume (stage B1) and 19 of 21 with greater than 4.0 cc in tumor volume (stages B2 and B3). All 10 of the stage B1 cancers were free of contralateral lobe capsular penetration while 1 of the 13 stage B2 nodules had minimal contralateral capsule penetration in the area of the neurovascular bundle. We believe that the modified nerve-sparing radical prostatectomy should be limited to the contralateral side in stage B disease.