BACKGROUND Although thyroid nodules are frequently detected in patients during routine examinations, such nodules are rarely malignant. Surgical treatment of nodules is controversial because of the possible complications associated with surgery, and there is an unmet need for a minimally invasive alternative. We previously reported on a high-intensity focused ultrasound (HIFU) device that induced necrosis in ewe thyroids. This complementary study on 27 ewes evaluated the use of the device to produce thyroid lesions, characterized the HIFU-induced lesions on the thyroid and surrounding structures, and evaluated the safety and reproducibility of the method. METHODS A spherical 3-MHz transducer that was coupled to a 5-MHz linear array ultrasound imaging probe was used to generate powerful acoustic waves to destroy thyroid tissue. Three series of experiments were conducted: thyroid lesion experiments (10 ewes), safety experiments (4 ewes), and reproducibility experiments (13 ewes). After fixation of the ewe's neck, tissue lesions were examined both macroscopically and histologically. RESULTS First, individual pulsed acoustical waves were used to induce lesions in 19 thyroid lobes. In most lesions, there was coagulative necrosis that was replaced later by fibrosis. Macroscopic examination of adjacent organs revealed skin lesions and muscle injuries. A second series of experiments evaluated the consequences of HIFU pulsed waves on structures surrounding the thyroid to better characterize possible side effects of HIFU. Firings at the periphery of eight lobes revealed macroscopic lesions in the trachea of one ewe and superficial esophagus lesions in three ewes. The recurrent nerves were damaged bilaterally in one ewe that died from dysphagia 3 days after HIFU. Four ewes were found to have muscle injuries, but no skin lesions were observed. A third series of experiments evaluated the reproducibility of a HIFU prototype designed specifically for human use. Thyroid lesions were obtained in 25 of the 26 treated lobes. No damage to the nerves, trachea, esophagus, or muscles was observed. About 3 of the 13 ewes had superficial skin burns. CONCLUSIONS The results obtained in the ewe model show that thyroid lesions with a defined volume can be induced safely and suggest that the HIFU device is now ready for evaluation in humans.