To evaluate the safety and efficacy of a minipercutaneous nephrolithotomy procedure (mini-PCNL) through a 20F sheath.Twenty-one patients underwent a mini-PCNL from October 1997 to October 1999. The mean number of calculi was 1.8 (range 1-9). The mean cross-sectional area of the stone burden was 2.8 cm2 (range 0.72-6.5 cm2). Nephrostomy tract dilation was performed with an 18F Cook Omega or Pursuit balloon dilation catheter (6-mm diameter, 10-cm length; 12 or 14 atm, respectively), and a 20F nephrostomy sheath was advanced over the inflated balloon.The mean operative time was 54 minutes. The mean decrease in hematocrit was 3.4%, and no patient required a blood transfusion. The mean percutaneous tube duration was 22 hours. The mean hospital stay was 1.1 days. The single-procedure stone-free rate was 90%. Two patients underwent successful SWL of residual middle-caliceal calculi that could not be accessed through the existing tract. There were no major complications, but there was one episode of prolonged fever secondary to atelectasis. Five patients remain free of radiographic or symptomatic recurrence after antegrade endopyelotomy for ureteropelvic junction obstruction after a mean of 11 months' follow-up.By using a 20F sheath rather than the traditional 30F sheath, we decreased the volume of renal parenchyma that is dilated by 56%. This change appears to decrease perioperative bleeding and pain as well as parenchymal scarring.