We have conducted a double-blind, randomized, placebo-controlled trial with oral diclofenac to study the prophylactic effect on renal colic recurrence and spontaneous stone expulsion rate. Forty-one patients were given 50 mg oral diclofenac 3 times a day for 7 days after being discharged for a colic episode from Oslo Emergency Hospital (< 24 h stay) and 39 patients were given matching placebo tablets. The number of new renal colic episodes per accumulated patient treatment days was 64/287 in the diclofenac group and 119/273 in the placebo group (p < 0.01). This difference was greatest during the first 4 treatment days. A similar trend was found for pain intensity (0-10 cm VAS) with the greatest difference on day 1 (4.3 vs. 2.8, p = 0.05). Side effects, mainly gastrointestinal, were reported for 14% of the treatment days in both treatment groups. Stone expulsion rate was almost identical (28 vs. 29 days), regardless of stone size. Readmission rate to Oslo Emergency Hospital/other hospitals were 10 and 67% (p < 0.001). In conclusion, oral treatment with diclofenac was effective as short-term prophylaxis of new colic episodes, especially during the first 4 days, and reduces the number of hospital readmissions significantly. The stone passage rate appears not to be affected.