The effect of high-intensity focused ultrasound (HIFU) ablation on the morphology of prostatic, renal and testicular tissue was studied by light and electron microscopy. Specimens were obtained in 21 patients 1 h to 10 weeks after lesioning. Histological findings showed consistent coagulative necrosis with precisely defined, sharp margins to normal tissue. Lesion size and position correlated well with the assumed target zones, suggesting that HIFU permits therapeutic tissue ablation.