Purpose : To evaluate the efficacy and safety of sono-guided ethanol sclerotherapy for benign thyroid cysts.Materials and Methods : We studied 22 patients with benign thyroid cysts (9 pure and 13 complex) confirmed by fineneedle aspiration biopsy. The procedure was performed every one or three months for a total of one or twosessions(mean 1.2 times). Sono-guided needle aspiration of the cysts was followed by instillation of absoluteethanol(99.9%) into the cystic cavity ; the injected volume of ethanol was 40-100% that of aspirated fluid.Follow-up ultrasonography was performed one to ten months( mean: 3.5 months) after the final session, andcomplications occurring after ethanol sclerotherapy were observed. Results : Initial cystic volume was3.5-42(mean: 13)ml. In 21 patients( 95%), cystic volume had decreased: In 13 patients(59%), the decrcase was 50%or more, and in six (27%) was 1-49%. In two patients(9%), the cysts had been obliterated, but in one there was nochange. The volume of instilled ethanol correlated significantly with the volume reduction rate of the cyst (p <0.01, t test). After ethanol sclerotherapy, the volume of thyroid cysts larger than 10ml had significantlydecreased compared to that of thyroid cysts with a volume less than 10 ml(p < 0.005, t test). Importantlongstanding and severe complications were not observed ; in two patients, a small amount of leakage insubcutaneous tissue caused local pain and redness. Conclusion : Sono-guided ethanol sclerotherapy is a safe andeffective tool for the treatment of benign thyroid cysts.