Biologic aggressiveness of palpable and nonpalpable prostate cancer: assessment with endosonography. The biologic aggressiveness of palpable versus nonpalpable prostate cancer was evaluated in 666 patients studied with endosonography over a 24-month period. Biologic aggressiveness was defined by a combined histologic and grade-stage category score. In 314 patients suspected of having prostate cancer 328 biopsies were performed. Carcinoma was detected in 99 patients, by means of both palpation and endosonography (n = 80), endosonography alone (n = 9), and palpation alone (n = 8); two cancers were not detected with either palpation or endosonography. All patients with normal results of digital examination had a combined grade-stage category score lower than 9. Fifty-five of 69 patients (80%) with abnormal results of digital examination and available histologic data had a histologic score of 6 or higher; 38 of these patients (69%) had a combined grade-stage category score of 9 or higher. Although the number of patients is small, these data suggest that nonpalpable cancers are biologically less aggressive than palpable ones and that the advantage of endosonography over palpation in detection of clinically significant cancers is limited. 