INTRODUCTION. This study examines the impact of the degree of interventricular septum reduction (<5 mm and ≥5 mm) on outcomes following alcohol septal ablation. METHODS AND MATERIALS. Data from 597 patients with obstructive hypertrophic cardiomyopathy who underwent alcohol septal ablation were analysed. Patients were divided into two groups: septal reduction <5 mm (329 patients); septal reduction ≥5 mm (268 patients). To correct the imbalance between the groups, the propensity score matching method was used. Outcomes were studied in both matched and unmatched cohorts. RESULTS. In the long-term follow-up period, 63 deaths were recorded. In the group with ≥5 mm reduction, gradients of the left ventricular outflow tract were lower (p<0.0001). Residual obstruction was more frequently observed in the <5 mm reduction group: unmatched cohort: 87 (26 %) vs. 37 (14 %), p<0.0001; matched cohort: 66 (33 %) vs. 21 (10 %), p<0.0001. Long-term survival was statistically significantly lower in the<5 mm reduction group: HR 0.47 (95 % CI: 0.24-0.89), p=0.019. CONCLUSION. Septal reduction of less than 5 mm following alcohol septal ablation is associated with: higher gradients of the left ventricular outflow tract; a higher incidence of residual obstruction; worse long-term survival.