INTRODUCTION: Peroral endoscopic myotomy (POEM) may result in a distended distal esophagus, referred to as a blown-out myotomy (BOM), the relevance of which is uncertain. The aim of this study was to investigate the prevalence, risk factors, and associated symptoms of BOM after achalasia treatment. METHODS: A data set of the locally treated patients in a randomized controlled trial comparing POEM with pneumatic dilation (PD) was analyzed. A BOM is defined as a >50% increase in esophageal diameter at its widest point in the distal esophagus between the lower esophageal sphincter and 5 cm above. RESULTS: Seventy-four patients were treated in our center, and 5-year follow-up data were available in 55 patients (32 patients [58%] randomized to POEM, 23 [42%] PD). In the group initially treated with POEM, the incidence of BOM increased from 11.5% (4/38) at 3 months, to 21.1% (8/38) at 1 year, 27.8% (10/36) at 2 years, and 31.3% (10/32) at 5 years. None of the patients treated with PD alone developed a BOM. Patients who developed a BOM had a higher total Eckardt score and Eckardt regurgitation component compared with patients who underwent POEM without BOM development (3 [2.75–3.25] vs 2 [1.75–3], P = 0.032, and 1 [0.75–1] vs 0 [0–1], P = 0.041). POEM patients with a BOM more often report reflux symptoms (85% [11/13] vs 46% [2/16], P = 0.023) and had a higher acid exposure time (24.5% [8–47] vs 6% [1.2–18.7], P = 0.027). DISCUSSION: Thirty percent of the patients treated with POEM develop a BOM, which is associated with a higher acid exposure, more reflux symptoms, and symptoms of regurgitation.