To evaluate percutaneous nephrolithotomy (PNL) and extracorporeal shock wave lithotripsy (ESWL) for their clinical effects, their cost effectiveness, their complication rates, and the patients' experiences, 55 consecutive patients were randomised to have one or other operation between October 1986 and October 1988. Six patients were excluded, 21 were treated with PNL and 28 with ESWL as primary treatment. Mean hospital stay and length of treatment were longer for PNL than for ESWL. Since 1 July 1987 all patients having ESWL have been treated without anaesthesia (n = 15), whereas epidural anaesthesia was used for all PNL. Slightly more of the ESWL patients experienced some pain during treatment. Minor complications or pain were more common after ESWL during the first 10 days after discharge from hospital. If patients with stone fragments of 4 mm or less were regarded as having a successful outcome, the success rates after one year were 94% for PNL and 77% for ESWL. The overall total cost was lower for ESWL than for PNL, the cost per successfully treated patient being 2172 pounds for PNL and 1810 pounds for ESWL. Medium sized kidney stones (6-30 mm, or 2-3 stones of 20 mm or less) can be efficiently and cheaply treated by both PNL and ESWL, though the cost of ESWL is lower. Even if effects other than cost (such as complications and patients' experience) are borne in mind, ESWL was superior to PNL for this group of patients.