Ultrasound-guided percutaneous ethanol injection (PEI) was first proposed by Livraghi in 1990 as a possible therapy for autonomously functioning thyroid nodules (AFTN). The procedure is performed on out-patients; is rapid; there is no need of anesthesia, nor of bed rest or patient observation after treatment. Under direct sonographic control a limited amount of 95% sterile ethanol (1-5 ml) is slowly injected into the nodule. In predominantly cystic nodules complete fluid removal is preliminarily performed, and thereafter ethanol is injected on the basis of the aspirated fluid volume without removing the needle. PEI-induced thyroid damage is characterized by coagulative necrosis and haemorrhagic infarction due to vascular thrombosis, and is well defined from the surrounding thyroid parenchyma. Several papers have confirmed the effectiveness of PEI. Normalization of serum TSH and thyroid hormones, marked decrease of nodule volume, and effacement of the previously hyperfunctioning area at thyroid scintiscan are reported in 64-85% of toxic AFTN. Nontoxic AFTN are successfully treated in 80-100% of the cases. PEI significantly superior to aspiration alone in inducing volume reduction of benign cystic thyroid nodules, and the recurrence rate is only 2.5-5%. Complications requiring hospitalization were not reported, but the importance of experienced operators must be stressed to avoid transient dysphonia due to recurrent laryngeal nerve damage. Prior to PEI malignancy must be ruled out by fine needle aspiration in all patients. Radioiodine and surgery remain the treatments of choice of large toxic thyroid nodules, but PEI can be effectively used in selected cases.(ABSTRACT TRUNCATED AT 250 WORDS)