Ureteral colic occurs in 24 to 34 per cent of all patients following extracorporeal shock wave lithotripsy. Recent research has shown prostaglandin synthetase inhibitors to be effective in relieving the pain associated with ureteral colic. Our prospective, controlled, double-blind, randomized study was designed to test the efficacy of indomethacin in the prophylactic treatment of pain after extracorporeal shock wave lithotripsy. Patients undergoing extracorporeal shock wave lithotripsy were randomized into 2 groups. Group 1 received 100 mg. indomethacin suppositories twice daily and group 2 received placebo suppositories. After extracorporeal shock wave lithotripsy 2 analgesics were available to the patients: oral co-dydramol or intramuscular pethidine was offered in the normal manner by the nursing staff. The pre-extracorporeal shock wave lithotripsy x-ray was used to make a quantitative estimate of the total stone burden in each patient. The post-extracorporeal shock wave lithotripsy analgesic requirement was used to compare the 2 groups. Of 112 patients recruited to the study 55 received indomethacin and 57 received placebo. The request for analgesia in the 2 groups was not different (28 of 55 and 33 of 57, respectively). However, in the indomethacin group only 6 patients required pethidine (10 doses), compared to 18 (41 doses) in the placebo group. This difference is statistically significant (p less than 0.01). There was no difference between the 2 groups in the occurrence of ureteral steinstrasse. Indomethacin has been shown to be effective in the prophylactic treatment of ureteral colic after lithotripsy.