Objectives To evaluate irrigant flows and intrapelvic pressures with small-diameter access sheaths. Ureteral access sheaths improve irrigant flow and decrease intrarenal pelvic pressures during flexible ureteroscopy. However, no comparisons of individual sheaths have been conducted. Previous studies have demonstrated more favorable results with the 12F sheath than with the 10F sheath. Methods Ureteral access sheaths were tested ex vivo in porcine kidneys. An 18F angiocatheter was placed in the renal pelvis and connected to a Hewlett Packard Gauss Pressure transducer. Irrigant was maintained at 100 mm Hg pressure. Irrigant flow and intrapelvic pressures were measured with three flexible ureteroscopes at baseline and using each of four 10F sheaths, with the sheaths positioned in the middle ureter and the ureteroscopes positioned in the renal pelvis. The pressure at which irrigant efflux through the sheath occurred and the rate of irrigant efflux through the access sheath were measured. Results Intrapelvic pressures measured greater than 40 mm Hg, and irrigant flows remained at less than 15 mL/min when the Olympus URF-P3 and Storz 11274AAU flexible ureteroscopes were tested with all four sheaths. The intrapelvic pressures, irrigant inflow, and irrigant efflux with the Wolf 7325.172 (7.5F) flexible ureteroscope were optimized in combination with the Cook Peelaway 10F and Applied Access 10F sheaths. Conclusions Small ureteral access sheaths should be used only with the Wolf 7325.172 flexible ureteroscope. The Cook Peelaway (10F) and Applied Access (10F) sheaths offered the greatest increase in irrigant flow and decrease in intrapelvic pressures.