BACKGROUND: EUS response assessment in patients with locally advanced esophageal cancer undergoing neoadjuvant chemoradiation therapy (CRT) is limited by disintegration of the involved anatomic structures. OBJECTIVE: Predictive and prognostic values of a prospectively defined maximum tumor thickness (MTT). DESIGN: Prospective open-label phase ll study (SAKK 75/02). SETTING: Multicenter, nationwide. PATIENTS: Of 66 patients with primary CRT, 56 underwent en bloc esophagectomy. INTERVENTIONS: EUS-measured MTT before and 2-5 weeks after CRT (yMTT). MAIN OUTCOME MEASUREMENTS: Cutoffs: (1) absolute thickness (yMTT) after CRT ≤ 6 mm; (2) relative reduction compared with baseline (ratio yMTT/MTT) ≤ 50%. Correlation between EUS measurements and histopathologic tumor regression grade (TRG) and overall survival (OS). RESULTS: Sixteen of 56 patients were not included for EUS evaluation (10 severe stenosis, 5 MTT not measured, 1 intolerance to second EUS). Characteristics (n = 40) were as follow: median age, 60 years; squamous cell carcinoma, 42%; and adenocarcinoma (AC), 58%. Initial stage was: 10 T2N1, 3 T3N0, 26 T3N1, 1 T3Nx; 14 of 23 AC Siewert type 1. Wilcoxon rank sum test showed significant correlation of TRG1 with yMTT ≤ 6 mm (P = .008) and yMTT/MTT ≤ 50% (P = .003). The effect of yMTT on TRG1 was significant (P = .0193; odds ratio, 0.687 [95% CI, 0.502-0.941]). The predefined cutoff of ≤ 6 mm for yMTT was predictive for TRG1 (P = .0037; Fisher exact test). After a median follow-up of 28.6 months, there was a clear trend for benefit in OS with yMTT ≤ 6 mm and yMTT/MTT ≤ 50%. LIMITATIONS: Small sample size. CONCLUSION: In a multicenter setting, MTT measured by EUS after CRT was highly predictive for response and showed a clear trend for predicting survival.