Thirty patients with partial or total staghorn lithiasis or calculi larger than 30 mm were treated by piezoelectric lithotripsy (PEL) monotherapy using an EDAP LT-01 lithotripter with ultrasound guidance. Nineteen of these patients had a pelvic stone; the other 11 had partial (nine) or total (two) staghorn lithiasis. All patients first underwent an initial lithotripsy session. No anesthesia or intravenous sedation was required in any case. If stone fracturization was achieved after this first session, a double J stent was inserted before the second lithotripsy session. Prior to the first session, 18 of 30 patients had a sterile urine culture; 12 of 30 presented major distention of the excretory tract. Results were analyzed to determine those factors influencing the outcome of this therapy. Three months after the first session, patients were considered cured if their stone had completely disappeared on abdominal plain films (14/30 = 46%). In seven patients (23.3%), fracturization had occurred but residual fragments remained (1-3 fragments less than or equal to 4 mm). No fracturization was obtained after the first session in nine patients (30.7%) (one total staghorn lithiasis, eight pelvic stones). The mean number of treatment sessions was five (range 1-15). Complications occurred in only 10% of patients (3/30): two steinstrasse and one acute pyelonephritis. Eighty-three percent of patients without major excretory tract distention and 55% of patients whose initial urine culture was sterile, achieved a stone-free state.(ABSTRACT TRUNCATED AT 250 WORDS)