Objective: Interlaminar endoscopic lumbar decompression is an increasingly used minimally invasive technique for treating lumbar disc herniation and lumbar spinal canal stenosis. However, technical difficulty varies substantially across cases because of anatomical and patient-related factors. This prospective study evaluated clinical and radiological predictors of surgical ease using an objective Endoscopic Ease Index (EEI). Methods: This prospective observational study included 122 patients who underwent interlaminar endoscopic lumbar decompression between January 2021 and May 2025. Preoperative clinical data and magnetic resonance imaging morphometric parameters—including interlaminar window dimensions, ligamentum flavum (LF) thickness, facet hypertrophy, and canal stenosis—were analyzed. Intraoperative difficulty was quantified using the EEI, which incorporates operative time, docking time, bleeding score, field clarity, and surgeon-rated difficulty. Predictors of difficult surgery (defined as the upper EEI tertile) were examined using multivariable logistic regression. Results: Of 122 cases, 41 (33.6%) were classified as difficult. Difficult cases had significantly narrower interlaminar windows (10.8±1.9 mm vs. 13.6±2.4 mm, p<0.001), greater LF thickness (5.0±0.9 mm vs. 3.6±0.7 mm, p<0.001), and a higher prevalence of facet hypertrophy (58.5% vs. 27.1%, p=0.004). In the multivariable model, 3 variables independently predicted difficult surgery: LF thickness >4 mm (odds ratio [OR], 5.22; p<0.001), interlaminar height <11 mm (OR, 4.48; p=0.001), and facet hypertrophy ≥ grade 2 (OR, 3.27; p=0.02). The predictive model demonstrated excellent discrimination (area under the receiver operating characteristic curve, 0.87). Despite greater operative complexity, postoperative improvements in pain (visual analogue scale) and disability (Oswestry Disability Index) were comparable between groups. Conclusion LF hypertrophy, reduced interlaminar height, and facet hypertrophy independently predict greater technical difficulty during interlaminar endoscopic lumbar decompression. Preoperative identification of these morphometric parameters may improve surgical planning and help optimize outcomes.