BACKGROUND & AIMS: Many patients with persistent dysphagia and regurgitation after therapy have low or no lower esophageal sphincter (LES) pressure. Distensibility of the esophagogastric junction (EGJ) largely determines esophageal emptying. We investigated whether assessment of the distensibility of the EGJ is a better and more integrated parameter than LES pressure for determining efficacy of treatment for patients with achalasia. METHODS: We measured distensibility of the EGJ using an endoscopic functional luminal imaging probe (EndoFLIP) in 15 healthy volunteers (controls; 8 male; age, 40 ± 4.1 years) and 30 patients with achalasia (16 male; age, 51 ± 3.1 years). Patients were also assessed by esophageal manometry and a timed barium esophagogram. Symptom scores were assessed using the Eckardt score, with a score <4 indicating treatment success. The effect of initial and additional treatment on distensibility and symptoms was evaluated in 7 and 5 patients, respectively. RESULTS: EGJ distensibility was significantly reduced in untreated patients with achalasia compared with controls (0.7 ± 0.9 vs 6.3 ± 0.7 mm(2)/mm Hg; P < .001). In patients with achalasia, EGJ distensibility correlated with esophageal emptying (r = -0.72; P < .01) and symptoms (r = 0.61; P < .01) and was significantly increased with treatment. EGJ distensibility was significantly higher in patients successfully treated (Eckardt score <3) compared with those with an Eckardt score >3 (1.6 ± 0.3 vs 4.4 ± 0.5 mm(2)/mm Hg; P = .001). Even when LES pressure was low, EGJ distensibility could be reduced, which was associated with impaired emptying and recurrent symptoms. CONCLUSIONS: EGJ distensibility is impaired in patients with achalasia and, in contrast to LES pressure, is associated with esophageal emptying and clinical response. Assessment of EGJ distensibility by EndoFLIP is a better parameter than LES pressure for evaluating efficacy of treatment for achalasia.