Background: For clinical stage I submucosal (cT1b-SM) ESCC, surgery is the standard treatment and CRT is optional. We conducted a single-arm confirmatory study of diagnostic ER plus selective CRT for cT1bN0M0 ESCC and reported the 3-year survival at 2016 ASCO Annual Meeting. We will report the final data of survival analysis after 5-year follow-up with a cutoff date of Aug 2017. Methods: Clinical stage I ESCC (cSM1-2, N0M0), tumor size ≤ 5 cm and circumference ≤ 3/4 was eligible. After ER, additional treatment was selected based on the histological evaluation: Group A, pT1a with negative resection margin and no lymphovascular invasion (LVI) -no additional treatment; Group B, pT1b with negative resection margin and pT1a with LVI -prophylactic CRT; Group C, pT1b with positive resection margin -definitive CRT. CRT consisted of concurrent 2 courses of chemotherapy (5-fluorouracil and cisplatin with 4-week interval, and radiotherapy of 41.4 Gy/23 fr (Group B) or 50.4Gy/28 fr (Group C). Primary endpoint was 3-year overall survival (OS) of Group B. The sample size was 82 for primary analysis, with one-sided alpha of 0.05 and power of 90%, based on the expected and threshold 3-year OS as 90% and 80%. Final analysis was planned after 5-year follow-up for all pts. Results: Between Dec 2006 and July 2012, 177 pts were enrolled from 23 institutions in Japan. 176 pts underwent ER and Group A/B/C were 74/87/15, respectively. The 3- and 5-year OS of Group B was 90.8% (90% CI; 84.1- 94.8) and 89.7% (95% CI; 81.1-94.5). The 3- and 5-year OS of all pts was 92.6% (90% CI; 88.6-95.3) and 90.9% (95% CI; 85.6-94.3). Twenty pts relapsed (Group A; 1 primary, 1 distant LN, Group B; 4 primaries , 8 regional LNs, 2 distant, Group C; 2 regional LNs, 2 distant), 7 pts underwent salvage esophagectomy. Univariable analysis in 83 pts of Group B showed that vascular invasion , one course of chemotherapy, SM2 with LVI had lower progression-free survival. Conclusions: Five-year survival data was comparable to that of surgery or CRT for c stage I ESCC. Vascular invasion, one course of chemotherapy, SM2 with LVI may be a risk factor of recurrence for prophylactic CRT after ER. Clinical trial information: UMIN000000553.