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We assessed the approachability of the upper calyx through lower calyx access for prone and supine percutaneous nephrolithotomy and used computerized tomography to analyze anatomical factors that may influence it.A prospective series of 45 patients treated with percutaneous nephrolithotomy were operated on in the prone (20) and supine (25) positions. Computerized tomography simulated access to the lower and upper calyx longitudinal axes were used to measure skin-to-lower calyx distance, thickness of the body wall, muscle and fat, the muscle-to-fat thickness ratio and the angle between the lower calyx tract and the upper calyx axis. Intraoperative approachability to the upper calyx was also evaluated.The upper calyx was successfully approached in 20% of prone and 80% of supine percutaneous nephrolithotomies (p <0.0001). The average skin-to-lower calyx distance was 98.4 mm (range 65.3 to 128.6) in the prone position and 98.7 mm (range 60.8 to 150) in the supine position (p = 0.99). Body wall and muscular thickness, and the muscle-to-fat thickness ratio were significantly lower in supine than prone nephrolithotomy (p <0.001, <0.0005 and <0.05, respectively). The average angle between the lower and upper calyces axes was wide in the supine position (141 degrees, range 90 to 170) and acute in the prone position (84 degrees, range 65 to 110, p <0.05E-10).Upper calyx endoscopic approachability through the lower calyx is significantly higher in supine than in prone percutaneous nephrolithotomies, possibly due to a thinner body wall, a thinner muscular layer, a lower muscle-to-fat thickness ratio and a wider angle between the lower and upper calyx axes.