Thirty patients with partial of total staghorn calculi or pyelic calculi greater than 30 mm were treated by extracorporal piezo-electric lithotripsy (PEL) exclusively with an EDAP LT 01 lithotripter equipped with an ultrasound localisation system. Nineteen patients had a pyelic calculus and the others a partial (n = 9) or total (n = 2) staghorn calculus. All patients first underwent extracorporal lithotripsy (ECL). None of the patients received IV sedation or anesthesia. When fragmentation of the calculus was observed after the first session, a double J stent was inserted before the second ELC session. Before the first session, urine samples were sterile in 18 of the 30 patients; 12 of the 30 patients presented major distention of the urinary tract. Results were analysed to identify factors affecting results of this type of treatment. Patients whose calculus had completely disappeared on plain films three months after the first session were considered to be cured clinically and radiologically (14/30 = 46%). Seven patients (23.3%) were clinically cured (absence of pain and sterile urine) but there were residual fragments (1 to 3 fragments less than or equal to 4 mm). No fragmentation was obtained after the first session in 9 patients (30.7%) (1 total staghorn, 8 pyelic calculi). The mean number of sessions was 5 (range 1-15). Only 10% of the patients (3/30) presented a complication: 2 steinstrasses and 1 acute pyelonephritis. 83% of the patients without urinary tract distention and 55% of the patients whose urines were initially sterile were cured.(ABSTRACT TRUNCATED AT 250 WORDS)