Since March 1985, over 1000 patients have been treated on the lithotripter at St Thomas' Hospital. Since it is the only machine in the country offering treatment to National Health Service patients at no cost to the referring Health Authority, there has been a heavy demand for treatment and 97% of referrals have been accepted. Analysis of the first 1000 patients shows extracorporeal shockwave lithotripsy (ESWL) to be a safe procedure with a low morbidity rate and no mortality. The number of patients who were stone-free 3 months after treatment was low (44.1%) compared with the numbers reported in other series. The most likely reasons for this are the poor follow-up rate (48.9%), the stringent criteria for the diagnosis of "stone-free" and a possible skewed referral and follow-up pattern.