TPS506 Background: Which is superior to treat hepatocellular carcinoma (HCC), surgery or percutaneous ablation? This has been one of the most important questions in clinical research of hepatology. However, it has long remained unanswered because it is difficult to approach, as we mentioned previously (Ann Surg 2008;247:557-8). Thus, in 2009, we started this multicenter study in Japan to reach a strong conclusion to the above question (UMIN:000001795). Methods: The major inclusion criteria are: tumor foci numbering three or fewer, and each measuring 3 cm or less); Child–Pugh score of 7 or less; aged between 20 and 79 years; and indications for either surgical resection or radiofrequency ablation (RFA), as described previously (Hepatol Res 2010;40:851-2). In the study design, we adopted two co-primary end-points: overall survival (OS) and recurrence-free survival (RFS). If assuming 3-year RFS following surgery and RFA are assumed as 45% and 35%, respectively, 600 cases are regarded as necessary. Because 600 cases would be also sufficient to detect 10% difference in OS, the target number of cases was set as 600. However, the recruitment was very difficult. The Independent Data Monitoring Committee recommended us to transiently stop the recruitment at the end of August 2015 when the number of the registered cases reached 308. At submission of this abstract, we are considering whether the current number (308) is enough or not to detect 10% difference in RFS by calculating conditional predictive power. We will describe the trial design, and the current situations in detail for debate. Clinical trial information: UMIN:000001795.