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Introduction EndoFLIP is increasingly used to characterise oesophagogastric junction (EGJ) physiology in achalasia, but optimal procedural targets and clinically meaningful distensibility index (DI) thresholds remain uncertain. We evaluated the effect of peroral endoscopic myotomy (POEM) on EGJ distensibility and explored whether DI is associated with oesophageal emptying on timed barium oesophagram (TBE).Methods We conducted a retrospective observational study of patients undergoing POEM at a tertiary centre. EndoFLIP measurements were recorded at baseline and immediately post-POEM. DI was assessed at 60 mL fill volume and EGJ diameter at 70 mL. Paired baseline and post-POEM measurements were compared using the Wilcoxon signed-rank test. For all available EndoFLIP–TBE pairs (baseline and post-POEM), the association between DI and 5-minute TBE column height (<5 cm vs ≥5 cm) was examined using Mann–Whitney U testing and receiver operating characteristic (ROC) analysis. Clinical outcomes at 3 months were assessed using the Eckardt score.Results Ninety-one patients underwent POEM. Paired baseline and post-POEM EndoFLIP data were available for 12 patients. Median DI increased from 1.6 mm 2/mmHg (IQR 1.18–1.70) at baseline to 6.2 mm2/mmHg (IQR 4.88–7.20) post-POEM (p=0.0005), while median EGJ diameter increased from 9.6 mm (IQR 8.48–11.5) to 16.1 mm (IQR 13.98–17.55; p=0.001). Across 21 combined EndoFLIP–TBE pairs, DI was significantly higher in studies associated with successful oesophageal emptying (TBE <5 cm) compared with impaired clearance (median 5.9 vs 2.0 mm2/mmHg; p=0.009). DI demonstrated good discrimination for TBE clearance (AUC 0.84), with an exploratory threshold of approximately 4.0 mm2/mmHg. All patients with post-POEM EndoFLIP and 3-month follow-up achieved clinical response (Eckardt ≤3).Conclusions POEM results in a significant improvement in EGJ distensibility and diameter on EndoFLIP. Higher DI values are associated with improved oesophageal emptying, with an exploratory DI threshold around 4 mm 2/mmHg. These preliminary findings support the physiological relevance of DI and warrant validation in larger prospective cohorts.