Apical invasion and positive apical margins were assessed in 165 consecutive radical prostatectomies. Apical invasion, defined as cancer in the distal 8 mm of the prostate, was evident in more than 80% of the cases, and apical margins occurred in 16% of the specimens with apical Clinical judgement was not effective in predicting apical cancer. Frequency of apical margins increased in proportion to greater cancer volume, from 9.8% in cancers smaller than 4 cc to 30.7% in cancers larger than 12 cc. However, most positive margins in the group with cancers smaller than 4 cc were caused by inadvertent incision into the prostate during the operation, whereas the vast majority of apical margins in cancers larger than 4 cc were caused by capsule penetration of the tumor. Although margins associated with capsule penetration occurred characteristically in the posterior (rectal) portion of the apex, margins caused by incision into the prostate were distributed over the entire apical surface of the gland. Positive margins at the urethral stump were quite uncommon (occurring in four cases). These findings suggest that modifications of surgical technique might reduce the frequency of this complication.