PURPOSE: The purpose of this study was to understand the principle of percutaneous nephrolithotomy(PNL), toincrease its success rate, and to base the radiologic approach on preoperative interventional tract dilatation. MATERIALS AND METHODS: Twenty-six patients(male, 19, female, 7) with staghorn or pelviocalyceal calculi who hadundergone PNL were included in this study. After percutaneous nephrostomy(PCN) was performed in each patient, thetract was dilated to 30F with Amplatz dilators using coaxial techniques or a 30F, 10cm high-pressure tractdilatation balloon. One day before the calculi were fragmented and removed by PNL, a 30F Amplatz sheath wasinserted under local anesthesia. The time required for tract dilatation, complications, and surgery was analyzed. RESULTS: Percutaneous tract dilatation were successful in all patients, and forceps, a nephrolithotome and anephroscope were successfully introduced into the pelvocalyceal system. The average procedure time was 19.5minutes ; a double J stent was replaced in the pelvocalyceal system of 13 of 26 patients (50%) ; because ofimproper catheter direction or/and bleeding, the Amplatz sheath was replaced in three. After PNL, complicationsinvolving renal hematoma occurred in one patient. This did not affect the patient's prognosis, however. CONCLUSION:Owing to its short procedure time and low complication rate, interventional preoperative tract dilatation forPNL is thought to be an effective technique for patients with staghorn or pelvocalceal calculi.