PURPOSE: To evaluate the ability of percutaneous ethanol ablation in the treatment of benign cysts of the liver and kidney, and to decide the need and the time of retreatment when the cysts remain on the follow-up ultrasonogram. MATERIALS AND METHODS: Twenty benign cysts(8 hepatic and 12 renal cysts) in 18 patients diagnosed or confirmed either by ultrasound, CT or cytology were treated with percutaneous ethanol injection(PEI). After evacuation of cystic fluid, 15-900ml(amount corresponding to 40-50% of the volume of aspirated fluid) of absolute ethanol(99.9%) was injected into the cysts through the aspiration catheter. rln large cysts, two or more PEIs were done in one session. Follow-up ultrasonographic studies during the period of 12 months with 1-2 months interval after PEI were performed for evaluation of the therapeutic effect. RESULTS: Nine cysts(45%) disappeared completely within 2 months after initial PEI. Although 11 cysts(55.5%) recurred 2 months after initial PEI, 8 of them disappeared within 6 months and one within 8 months after inital PEI without additional PEI. As a result, 18 of 20 cysts(90%) disappeared within 8 months after initial PEI and most of the recurrent cysts disappeared within 6 months without additional PEI. No major complications were encountered concerning PEI, although transient abdominal pain, elevation of body temperature, and drowsiness were noted in 8 patients. CONCLUSION: PEI is an effective and safe modality for the treatment of benign hepatic or renal cysts and the apparent recurrence within 6 months after initial PEI might be mostly a transient, reactive or inflammatory fluid collection rather than real recurrence.