Positive margins were analyzed in 189 clinical stage B radical retropubic prostatectomies. Margins were identified by serially blocking the entire specimens in planes selected for optimum demonstration of capsule surface. Positive margins were divided into 2 categories: 1) those associated with capsular penetration of cancer and 2) those caused by inadvertent surgical incisions through the capsule into intracapsular cancer. Data were analyzed separately at each of 6 anatomical sites. Frequency of positive margins was related to the volume of cancer. Cancer of greater than 12 cc constituted a distinctive category in which seminal vesicle invasion, lymph node metastases and multiple positive margins were found in the majority of cases, signifying minimal possibility of cure. However, 31 positive margins occurred among 136 patients (23%) who were potentially curable by the criteria of normal seminal vesicles and absence of pelvic lymph node metastases; 17, of these 31 surgically positive margins (55%) occurred at the apex. Positive capsular penetration margins at the apex were volume-related, while negative margins were not. Site specific recommendations for avoiding positive and negative capsular penetration margins are suggested. The prostate apex, rectal and lateral surfaces, bladder neck and superior pedicles accounted for 48, 24, 16 and 10% of all positive margins, respectively. Dissection of the apical prostate and Denonvilliers' fascia require modifications of current surgical techniques if positive margins are to be avoided. Serum levels of prostate specific antigen may require as long as 5 years to become detectable when only 1 positive margin is the only evidence of nonorgan-confined disease.