Introduction: Peroral endoscopic myotomy (POEM) is a newly emerging treatment modality for achalasia as compared to conventional treatment like pneumatic dilatation (PD). Previous PD may make subsequent dissection and separation of tissue planes more challenging due to fibrosis. There is lack of a systematic study analyzing if POEM can be applied to patients with failed PD. We conducted this study to determine the safety and efficacy of POEM in the setting of prior PD. Methods: Medical records all the achalasia patients who underwent POEM at our institution between April 2014 and December 2017 were reviewed. Patients who underwent timed barium esophagram (TBE) and high-resolution manometry (HREM) prior to POEM along with two-month post-POEM esophageal pH study, TBE and HREM were included. Patients were categorized into two groups as: no prior PD (treatment naive) and prior PD group. Patients in either group may have had prior botulinum injection, however those with prior Heller’s myotomy were excluded. Demographics, procedural details, Eckardt’s scores, TBE, HREM and pH study findings were compared between the two groups. Eckhardt’s symptom score of <3 was considered as successful palliation of symptoms. Analysis of covariance was performed to assess the association between PD groups and change in outcomes from pre-POEM to 2-month post-POEM follow-up while adjusting for possible confounders. Results: A total of 83 patients (no prior PD=57; prior PD=26) met the study criteria. There were no significant differences in the age, gender, ethnicity, BMI, type of achalasia, myotomy length, operativetime, length of stay or complication rates between the two groups (table-1). Subjects with prior PD were likely to have longer duration of achalasia symptoms (p=0.029). There was a significant improvement in Eckardt score, TBE and HREM parameters for both groups (figure 1,2). On adjusted analysis, there was no significant difference in the pre-post POEM change in Eckardt score, TBE and HREM parameters amongst both groups. In the post-POEM pH study, proportion of patients with GERD symptoms, total abnormal acid exposure and abnormal DeMeester scores were similar in both groups. Conclusion: POEM appears to be a safe and effective treatment modality in achalasia patients with prior PD. Prior dilatation does not seem to have any obvious influence on the efficacy of POEM. POEM might emerge as the preferred approach for myotomy in patients with failed PD.318_A Figure 1. Patient characteristics, Procedure details and Outcomes318_B Figure 2. Comparison of Eckardt’s score and high-resolution manometry parameters (HREM) in patients with no prior pneumatic dilatation (PD) versus those with prior PD318_C Figure 3. Comparison of timed barium esophagram parameters (TBE) in patients with no prior pneumatic dilatation (PD) versus those with prior PD