We have conducted a double-blind randomised placebo controlled trial with oral diclofenac to study the prophylactic effect on recurrence of renal colic and rate of spontaneous stone expulsion. 41 patients were given 50 mg oral diclofenac three times a day for seven days after being discharged for a colic episode from Oslo Emergency Hospital and 39 patients were given matching placebo tablets. The number of new ureteral colic episodes per accumulated patient treatment days was 64/287 in the diclofenac group and 119/273 in the placebo group (p < 0.01). The difference was greatest during the first four days of treatment. A similar trend was found for pain intensity, with the greatest difference on day one. There was no difference in reported type or frequency of side effects in the two treatment groups. Stone expulsion rate was almost identical. The effect of the treatment was not affected by fluid intake. Re-admission rates to Oslo Emergency Hospital or other hospitals were 10 and 67% (p < 0.001).