At present, outpatient extracorporeal shock wave lithotripsy (ESWL) is practiced in many centers for the treatment of urinary calculi. The safety of outpatient treatment is difficult to assess, no appropriate indications and criteria for patient selection have been established. In a retrospective study 1,203 patients, treated with ESWL using the Dornier HM-3 lithotriptor for the first time over the year 1986, were evaluated to analyze possible risk factors for outpatient ESWL. By multivariate analysis various pretreatment factors were investigated for their relation with complications arising from ESWL therapy. It was shown that stone size and sex of the patient were the strongest determining factors. In the present study a low-risk group (5% complications) could be identified, suitable for outpatient treatment, corresponding to 74% of the patient population. The remaining patients (26%) are at higher risk (18% complications) and should be considered for hospital treatment.