In a phase I clinical trial the morphological impact and safety of high intensity focused ultrasound administered transrectally for tissue ablation in prostates from 22 patients undergoing subsequent prostatectomy were evaluated. Location and size of the tissue lesions correlated well with the predefined target area and revealed sharply delineated coagulative necrosis in all cases. Intervening tissues, such as the rectal wall and posterior prostate capsule, were invariably intact. In a subsequent phase II clinical trial the effectiveness of transrectal high intensity focused ultrasound as a novel minimally invasive treatment modality for 50 patients with symptomatic benign prostatic hyperplasia was determined. The maximum urinary flow rate (ml. per second) increased from 8.9 +/- 4.1 to 12.7 +/- 6.4 at 3 months in 44 patients, 12.4 +/- 5.6 at 6 months in 33 and 13.1 +/- 6.5 at 12 months in 20. During the same period the post-void residual volume (ml) decreased from 131 +/- 120 to 48 +/- 41, 59 +/- 42 and 35 +/- 30, respectively, and the American Urological Association symptom score (points) decreased from 24.5 +/- 4.7 to 13.3 +/- 4.4, 13.4 +/- 4.7 and 10.8 +/- 2.5, respectively. These data demonstrate that transrectal high intensity focused ultrasound is capable of inducing coagulative necrosis in the human prostate via a transrectal approach while preserving intervening and adjacent tissue. A 47% (+4.2 ml. per second) improvement in uroflowmetry and a 53% (-13.7 points) decrease in the American Urological Association symptom score 1 year after treatment clearly prove that transrectal high intensity focused ultrasound is a novel and safe minimally invasive treatment option for benign prostatic hyperplasia.