Objective To evaluate the impact of successful percutaneous balloon mitral valvuloplasty (BMV) on left atrial (LA) reservoir function and LA volume in patients with severe mitral stenosis (MS) using peak atrial longitudinal strain (PALS). Method This was a prospective, non-randomized observational study conducted at the Laxmipat Singhania (LPS) Institute of Cardiology, Kanpur from August 2018 to February 2020 among patients with severe rheumatic MS undergoing BMV to assess LA reservoir function and its volume after BMV using PALS. Inclusion criteria were symptomatic severe rheumatic MS (NYHA ≥II), normal ventricular systolic function, and suitable valve morphology. Exclusion criteria were the coexistence of aortic valve involvement, left atrial appendage clot, mitral leak more than mild, pregnancy, hypertension, diabetes, and coronary artery disease. To assess LA reservoir function and its volume after BMV, PALS was used. LA was divided into six regions of interest and longitudinal strain curves of individual segments together with global strain were recorded. PALS was calculated at baseline 24 hours following the intervention, and at three months of follow-up. Result Successful BMV was performed in 260 patients (109 or 41.9% males and 151 or 58.1% females), resulting in significant improvement in mitral valve area (MVA) (0.89±0.11 cm2 vs. 1.83±0.3 cm2; p<0.001). The mean age of patients was 26.7±4.7 years; 214 (82.3%) patients were in normal sinus rhythm (NSR) while 46 (17.7%) had atrial fibrillation (AF). Significant improvement in PALS was noted immediately following the procedure (6.5±11.6% vs. 7.7±10.5%; p< 0.001) and it continued to improve at three months of follow-up (6.5±11.6% vs. 11.3±12.5%; p<0.001), which was 24% and 74% improvement from baseline respectively. Significant reduction in indexed left atrial (LA) volume was observed immediately following the procedure (56.8±14.3 ml/m2 vs 48.4±12.5 ml/m2; p=0.003), and at three months of follow-up (56.8±14.3 ml/m2 vs. 45.4±13.3 ml/m2; p=0.002). Those with AF had lesser improvement in PALS in comparison to those with NSR (60% vs. 84%; p=0.044) at three months of follow-up. At three months, the increase in PALS was also lower in patients with a history of stroke as compared to those without it (55% vs 80%; p=0.039). Both LA volume and indexed LA volume reduced significantly immediately at 24 hours and during follow-up. Conclusion LA reservoir function, as assessed by PALS, is reduced in patients with severe MS. It improved significantly within 24 hours following BMV and continued to improve at three months of follow-up. It is an underutilized modality among patients of MS for decision-making prior to intervention and to assess the effect of the intervention.