Objective: The objective of treatment for polycystic liver disease is to reduce the liver volume and reduce or resolve compression symptoms such as abdominal fullness and abdominal pain due to hepatomegaly. Liver cysts are treated internally by puncture and aspiration of the cyst contents or hepatic artery embolization and surgically by cyst fenestration or hepatectomy, but no clear consensus has been reached concerning their selection. We introduced monoethanolamine oleate (EO) sclerotherapy therapy for liver cysts in 1999 and reported its effectiveness. In this study, cases were added, and the results including those of long-term follow-up were evaluated. Subjects: Twenty-two patients (5 males and 17 females, mean age 65.2) who underwent EO infusion therapy for liver cysts between January 1999 and June 2011 were evaluated. Methods: Liver cysts were punctured under ultrasound guidance, and a 7Fr pigtail catheter was inserted. After aspirating the cyst contents, EO was infused, and a clamp was applied for 24 h. Then, the catheter was declamped, cyst contents were aspirated again, and the catheter was removed. After the treatment, the cyst size was measured, and the patients were followed up. Results: Eight simple cysts in 8 patients (simple cyst group) and 21 cysts in 14 patients with multiple cysts (polycystic liver disease group) were treated and followed up over a median of 78 months (0-203 months). The mean volume reduction rate was 99% in the simple cyst group and 91% in the polycystic liver disease group (p = 0.04). One procedural accident resulting in liver abscess formation was observed in 1 patient 1 week after discharge, and it required drain placement and antibiotic administration. While mild abdominal pain was observed in a few patients, it was resolved spontaneously under observation. Conclusion: EO infusion therapy achieves fairly high treatment response in the volume reduction (99%) and sustained shrinkage over long-term follow-up. Therefore, this is a breakthrough technique in the treatment of polycystic liver disease as well as simple cyst and should be a standard of care in the treatment of this disease.