Objective: To assess whether morphometric and clinical parameters influence accessibility to the upper calyx through a lower calyx tract during supine percutaneous nephrolithotomy (SPCNL). Patients and Methods: Data for 114 consecutive SPCNLs performed between June 2014 and March 2016 in an academic medical center were analyzed. Successful approachability was defined as rigid nephroscopic access to the upper calyx introduced through a lower calyx access as attested by direct visualization of the upper calyx papillae and position of the nephroscope on intraoperative pyelography. Demographic, morphometric, and clinical parameters were assessed. Analysis of variance, Fisher's exact, chi-square tests, and univariate and multivariate models were applied. Results: Successful approach was achieved in 94 (82%) patients. In univariate analyses previous kidney operations (p = 0.03), body weight (p = 0.04), and acute lower calyx access to the upper calyx axis angle (p = 0.01E-13) correlated with failed access. Multivariate analyses revealed that an acute angle was the sole independent factor for failed access (odds ratio [OR] 1.4, p < 0.05; confidence interval [95% CI] 1.021, 2.035). Limitations include the inability to objectively determine the mobility of the kidney as a co-factor in successful approachability. Conclusions: SPCNL offers effective endoscopic accessibility through a lower calyx access. Previous kidney operations and body weight tend to negatively influence this accessibility. An acute angle between the lower calyx and the upper calyx is the single independent factor associated with failed accessibility.